python: Fix memleak in do_start_initialization

python: Fix memleak in do_start_initialization

While playing with valgrind, I noticed that with Python 3, the progname
variable in do_start_initialization is not being freed (concat returns a
malloc'ed string).  This patch uses unique_xmalloc_ptr to manage it.
With Python 2, we pass progname it directly to Py_SetProgramName, so it
should not be freed.  We therefore release it before passing it.

gdb/ChangeLog:

	* python/python.c (do_start_initialization): Change progname
	type to gdb::unique_xmalloc_ptr.  Release the pointer when using
	Python 2.