[builtin/printf] Remove usage of getenv() and unsetenv().

I couldn't detect this from the outside.  It didn't work in the OVM
build, and simpler code will translate more easily to C++.

build/oil-defs/Python-2.7.13/Modules/posixmodule.c/posix_methods.def

@@ -33,6 +33,7 @@ static PyMethodDef posix_methods[] = {
   {"isatty", posix_isatty, METH_VARARGS},
   {"pipe", posix_pipe, METH_NOARGS},
   {"putenv", posix_putenv, METH_VARARGS},
+  /*{"unsetenv", posix_unsetenv, METH_VARARGS},*/
   {"strerror", posix_strerror, METH_VARARGS},
 
   /* job control stuff */

core/state.py

@@ -614,7 +614,7 @@ def _InitVarsFromEnv(mem, environ):
   # variable.  Dash has a loop through environ in init.c
   for n, v in iteritems(environ):
     mem.SetVar(lvalue.Named(n), value.Str(v), scope_e.GlobalOnly,
-                flags=SetExport)
+               flags=SetExport)
 
   # If it's not in the environment, initialize it.  This makes it easier to
   # update later in MutableOpts.

osh/builtin_printf.py

@@ -354,12 +354,18 @@ def Run(self, cmd_val):
               #   variable `TZ'.  When the exported variable `TZ' is present,
               #   its value should be reflected in the real environment
               #   variable `TZ' before call of `tzset'.
+              #
+              # Note: unlike LANG, TZ doesn't seem to change behavior if it's
+              # not exported.
+              #
+              # TODO: In Oil, provide an API that doesn't rely on libc's
+              # global state.
+
               tzcell = self.mem.GetCell('TZ')
               if tzcell and tzcell.exported and tzcell.val.tag_() == value_e.Str:
                 tzval = cast(value__Str, tzcell.val)
                 posix.putenv('TZ', tzval.s)
-              elif posix.getenv('TZ'):
-                posix.unsetenv('TZ')
+
               time.tzset()
 
               # Handle special values:

spec/builtin-printf.test.sh

@@ -591,6 +591,34 @@ status=0
 status=1
 ## END
 
+#### %(strftime format)T doesn't respect TZ if not exported
+TZ=Asia/Tokyo  # NOT exported
+printf '%(%Y-%m-%d)T\n' 1557978599
+echo status=$?
+## STDOUT:
+2019-05-15
+status=0
+## END
+## N-I dash/mksh/zsh/ash STDOUT:
+status=1
+## END
+
+#### %(strftime format)T TZ in environ but not set in shell
+
+export TZ=Asia/Tokyo
+unset TZ  # unset in the shell, but still in the environment
+#repr TZ
+
+printf '%(%Y-%m-%d)T\n' 1557978599
+echo status=$?
+## STDOUT:
+2019-05-15
+status=0
+## END
+## N-I dash/mksh/zsh/ash STDOUT:
+status=1
+## END
+
 #### %10.5(strftime format)T
 # The result depends on timezone
 export TZ=Asia/Tokyo