main: Improve command-line handling and usage string

To determine program name from argv[0]/argv[1] remove everything after
the first '-' or '.' character. This allows names like snabbnfv,
snabbnfv-1.0, snabbnfv-lukes-turbo-charged-version, snabbnfv.linux,
and so on.

Make the usage printout for 'snabb' and 'snabb -h' and 'snabb --help' better:

    Usage: ./snabb <program> ...

    This snabb executable has the following programs built in:
      packetblaster
      snabbmark
      snabbnfv
      snsh

    For detailed usage of any program run:
      snabb <program> --help

    If you rename (or copy or symlink) this executable with one of
    the names above then that program will be chosen automatically.

The program list is determined automatically by the Makefile.

src/Makefile

@@ -182,13 +182,19 @@ $(RMOBJS): %: %.src
 	$(E) "MARKDOWN  $@"
 	$(Q) scripts/process-markdown $< > $@
 
-$(INCOBJ): obj/%_inc.o: %.inc Makefile | $(OBJDIR)
+$(INCOBJ): obj/%_inc.o: %.inc Makefile program/programs.inc | $(OBJDIR)
 	$(E) "INC       $@"
 	@(echo -n "return [=============["; \
 	 cat $<; \
 	 echo "]=============]") > $(basename $@).luainc
 	$(Q) luajit -bg -n $(subst /,.,$*)_inc $(basename $@).luainc $@
 
+# Create list of programs that exist
+program/programs.inc:
+	@ls -1d program/*/ | xargs basename -a > $@
+
+.FORCE: program/programs.inc
+
 # extra/ third party bits and pieces
 obj/strict.o: extra/strict.lua | $(OBJDIR)
 	$(E) "LUA       $@"

src/core/main.lua

@@ -33,43 +33,63 @@ function main ()
    zone("startup")
    require "lib.lua.strict"
    initialize()
-   local program, args = parse_command_line()
-   parameters = args
-   if not program then
-      print("Usage: snabb <PROGRAM> [ARGS]...")
-      os.exit(1)
+   local program
+   local args = parse_command_line()
+   if programname(args[1]) == 'snabb' then
+      -- Print usage when no arguments or -h/--help
+      if #args == 1 or args[2] == '-h' or args[2] == '--help' then
+         usage()
+         os.exit(1)
+      else
+         -- Strip 'snabb' and use next argument as program name
+         table.remove(args, 1)
+      end
    end
+   local program = table.remove(args, 1)
    if not lib.have_module(modulename(program)) then
-      print(programname(program) .. ": unrecognized program name")
+      print("unsupported program: "..programname(program))
+      print()
+      print("Rename this executable (cp, mv, ln) to choose a supported program:")
+      print("  snabb "..(require("program.programs_inc"):gsub("\n", " ")))
       os.exit(1)
+   else
+      require(modulename(program)).run(args)
    end
-   require(modulename(program)).run(parameters)
 end
 
--- programname("nfv-sync-master.2.0") => "nfv-sync-master"
+function usage ()
+   print("Usage: "..ffi.string(C.argv[0]).." <program> ...")
+   local programs = require("program.programs_inc"):gsub("[a-z]+", "  %1")
+   print()
+   print("This snabb executable has the following programs built in:")
+   print(programs)
+   print("For detailed usage of any program run:")
+   print("  snabb <program> --help")
+   print()
+   print("If you rename (or copy or symlink) this executable with one of")
+   print("the names above then that program will be chosen automatically.")
+end
+
+
+-- programname("snabbnfv-1.0") => "snabbnfv"
 function programname (program) 
-   return string.match(program, "([^/]+)$")
+   program = program:gsub("^.*/", "") -- /bin/snabb-1.0 => snabb-1.0
+   program = program:gsub("[-.].*$", "") -- snabb-1.0   => snabb
+   return program
 end
 -- modulename("nfv-sync-master.2.0") => "program.nfv.nfv_sync_master")
 function modulename (program) 
-   program = programname(string.gsub(program, "-", "_"))
+   program = programname(program)
    return ("program.%s.%s"):format(program, program)
 end
 
--- Return two values: program and parameters.
---
--- Program is the name of the program to run. For example 'snsh' or
--- 'packetblaster'.
+-- Return all command-line paramters (argv) in an array.
 function parse_command_line ()
-   local commandline = {}
+   local array = {}
    for i = 0, C.argc - 1 do 
-      table.insert(commandline, ffi.string(C.argv[i]))
-   end
-   local program = table.remove(commandline, 1)
-   if programname(program) == 'snabb' then
-      program = table.remove(commandline, 1)      
+      table.insert(array, ffi.string(C.argv[i]))
    end
-   return program, commandline
+   return array
 end
 
 function exit (status)