Make add-symbol-file's address argument optional

The (first) .text section must be always specified as the second
non-option argument.  The documentation states that GDB cannot
figure out this address by itself.  This is true if the object file
was indeed relocated, but it is also confusing, because all other
sections can be omitted and will use the address provided by BFD.

gdb/ChangeLog:
2018-06-28  Petr Tesarik  <ptesarik@suse.cz>

	* symfile.c (add_symbol_file_command, _initialize_symfile): Do not
	require the second argument.  If omitted, load sections at the
	addresses specified in the file.

gdb/doc/ChangeLog:
2018-06-28  Petr Tesarik  <ptesarik@suse.cz>

	* gdb.texinfo (Files): The address argument for "add-symbol-file"
	is no longer mandatory.

gdb/testsuite/ChangeLog:
2018-06-28  Petr Tesarik  <ptesarik@suse.cz>

	* gdb.base/relocate.exp: Test add-symbol-file behavior when the
	address argument is omitted.