Make '{add-,}symbol-file' not care about the position of command line arguments

Make '{add-,}symbol-file' not care about the position of command line arguments

This is a bug that's been detected while doing the readnever work.

If you use 'symbol-file' or 'add-symbol-file', the position of each
argument passed to the command matters.  This means that if you do:

  (gdb) symbol-file -readnow /foo/bar

The symbol file specified will (correctly) have all of its symbols
read by GDB (because of the -readnow flag).  However, if you do:

  (gdb) symbol-file /foo/bar -readnow

GDB will silently ignore the -readnow flag, because it was specified
after the filename.  This is not a good thing to do and may confuse
the user.

To address that, I've modified the argument parsing mechanisms of
symbol_file_command and add_symbol_file_command to be
"position-independent".  I have also added one error call at the end
of add_symbol_file_command's argument parsing logic, which now clearly
complains if no filename has been specified.  Both commands now
support the "--" option to stop argument processing.

This patch provides a testcase for both commands, in order to make
sure that the argument order does not matter.  It has been
regression-tested on BuildBot.

gdb/ChangeLog:

2017-12-01  Sergio Durigan Junior  <sergiodj@redhat.com>

	* symfile.c (symbol_file_command): Call
	'symbol_file_add_main_1' only after processing all command
	line options.
	(add_symbol_file_command): Modify logic to make arguments
	position-independent.

gdb/testsuite/ChangeLog:

2017-12-01  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.base/relocate.exp: Add tests to guarantee that arguments
	to 'symbol-file' and 'add-symbol-file' can be
	position-independent.