gdb: make gdbpy_parse_command_name return a unique_xmalloc_ptr

This avoids some manual memory management.

cmdpy_init correctly transfers ownership of the name to the
cmd_list_element, as it sets the name_allocated flag.  However,
cmdpy_init (and add_setshow_generic) doesn't, it looks like the name is
just leaked.  This is a bit tricky, because it actually creates two
commands (one set and one show), it would take a bit of refactoring of
the command code to give each their own allocated copy.  For now, just
keep doing what the current code does but in a more explicit fashion,
with an explicit release.

gdb/ChangeLog:

	* python/python-internal.h (gdbpy_parse_command_name): Return
	gdb::unique_xmalloc_ptr.
	* python/py-cmd.c (gdbpy_parse_command_name): Likewise.
	(cmdpy_init): Adjust.
	* python/py-param.c (parmpy_init): Adjust.
	(add_setshow_generic): Take gdb::unique_xmalloc_ptr, release it
	when done.

Change-Id: Iae5bc21fe2b22f12d5f954057b0aca7ca4cd3f0d