Fix add-symbol-file usage and errors

Fix add-symbol-file usage and errors

This patch updates add-symbol-file help and error text.

It changes add-symbol-file to throw an exception if "-s" is seen but
not all of the arguments are given.  Previously this was silently
ignored.

It changes the unrecognized argument message to more clearly state
what went wrong.

Finally, it updates the usage line in the help text to follow GNU
style regarding "metasyntactic variables"; a change I believe should
be made to all gdb help messages.

gdb/ChangeLog
2017-11-29  Tom Tromey  <tom@tromey.com>

	* symfile.c (add_symbol_file_command): Error if some arguments to
	-s are missing.  Change unrecognized-argument error message.
	(_initialize_symfile): Fix usage text for add-symbol-file.

gdb/testsuite/ChangeLog
2017-11-29  Tom Tromey  <tom@tromey.com>

	* gdb.base/relocate.exp: Update invalid argument test.
	Add new tests for invalid arguments.