Fix PR 21352: Command tsave does not support -r argument

Fix PR 21352: Command tsave does not support -r argument

This is an obvious fix for PR 21352.  The problem is that the argument
parsing loop is not using an "else if" where it should, and therefore
the '-r' option ends up unrecognized by GDB.

gdb/ChangeLog:
2017-04-05  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/21352
	* tracefile.c (tsave_command): Fix argument parsing for '-r'
	option.