gdb: make inferior::args a unique_xmalloc_ptr

Use unique_xmalloc_ptr to avoid manual memory management.

gdb/ChangeLog:

	* inferior.h (class inferior) <args>: Change type to
	unique_xmalloc_ptr.
	* inferior.c (inferior::~inferior): Don't free args.
	* infcmd.c (get_inferior_args): Adjust.
	(set_inferior_args): Adjust.

Change-Id: I96300e59eb2faf2d80660416a8f5694d243a944e