core.shm: Add alias() and children()

alias(toname, fromname) creates a new name for an existing
object. This is implemented as a symlink.

children() lists the names of objects under a path. This is a
directory listing.

The initial use for these features is providing links to apps. The app
has to learn what links it has using children() and each link it finds
is an alias of one created by the engine.

For example, the link 'a.tx->b.rx' would be one shm object created by
the engine with a symlink to make it available to each app:

    /engine/link/a.tx->b.rx  (real struct link created by engine)
    /app/a/link/output/tx -> /engine/link/a.tx->b.rx
    /app/b/link/input/rx  -> /engine/link/a.tx->b.rx

src/core/shm.lua

@@ -128,6 +128,20 @@ function unlink (name)
    return S.util.rm(root..'/'..path) -- recursive rm of file or directory
 end
 
+-- Return an array of objects under the prefix name.
+-- The names are returned unqualified e.g. 'x' and not 'foo/bar/x'.
+function children (name)
+   -- XXX dirtable returns an array but with a special tostring metamethod.
+   --     Potentially confusing? (Copy into plain array instead?)
+   return S.util.dirtable(root.."/"..resolve(name), true) or {}
+end
+
+-- Create an additional name for an existing object.
+function alias (toname, fromname)
+   assert(S.symlink(root.."/"..resolve(toname), root.."/"..resolve(fromname)),
+          "alias symlink failed")
+end
+
 function selftest ()
    print("selftest: shm")
    print("checking paths..")
@@ -145,10 +159,13 @@ function selftest ()
    print("create "..name)
    local p1 = map(name, "struct { int x, y, z; }")
    local p2 = map(name, "struct { int x, y, z; }")
+   alias(name, name..".alias")
+   local p3 = map(name..".alias", "struct { int x, y, z; }")
    assert(p1 ~= p2)
    assert(p1.x == p2.x)
    p1.x = 42
    assert(p1.x == p2.x)
+   assert(p1.x == p3.x)
    assert(unlink(name))
    unmap(p1)
    unmap(p2)
@@ -164,6 +181,7 @@ function selftest ()
    print(n.." objects created")
    for i = 1, n do unmap(objs[i]) end
    print(n.." objects unmapped")
+   assert((#children("/shm/selftest/manyobj/obj")) == n, "child count mismatch")
    assert(unlink("/"))
    print("selftest ok")
 end