Fix "set enum-command value junk"

Fix "set enum-command value junk"

With enum commands, we currently fail to notice junk after the value.

Currently:

  (gdb) set print entry-values compact foo
  (gdb) show print entry-values foo
  Printing of function arguments at function entry is "compact".

After this fix:

 (gdb) set print entry-values compact foo
  Junk after item "compact": foo

gdb/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* cli/cli-setshow.c (do_set_command) <var_enum>: Detect junk
	after item.