Fix "set integer-command unlimited junk"

With integer commands that support "unlimited", we currently fail to
notice junk after "unlimited":

 (gdb) show print elements
 Limit on string chars or array elements to print is 200.
 (gdb) set print elements unlimited foo
 (gdb) show print elements
 Limit on string chars or array elements to print is unlimited.

This commit fixes that.  After, we get:

 (gdb) set print elements unlimited foo
 Junk after "unlimited": foo

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

	* cli/cli-setshow.c (cli/cli-setshow.c): New parameter
	'expression'.  When parsing an expression, error out if there's
	junk after "unlimited".
	(parse_cli_var_uinteger, parse_cli_var_zuinteger_unlimited)
	(do_set_command): Adjust calls to is_unlimited_literal.

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

	* gdb.base/settings.exp (test-integer): Test junk after
	"unlimited".