Use unique_xmalloc_ptr for read_string

This changes read_string's "buffer" out-parameter to be a
unique_xmalloc_ptr, then updates the users.  This allows for the
removal of some cleanups.

I chose unique_xmalloc_ptr rather than byte_vector here due to the way
Guile unwinding seems to work.

Tested by the buildbot.

gdb/ChangeLog
2018-06-18  Tom Tromey  <tom@tromey.com>

	* valprint.h (read_string): Update.
	* valprint.c (read_string): Change type of "buffer".
	(val_print_string): Update.
	* python/py-value.c (valpy_string): Update.
	* language.h (struct language_defn) <la_get_string>: Change
	type of "buffer".
	(default_get_string, c_get_string): Update.
	* language.c (default_get_string): Change type of "buffer".
	* guile/scm-value.c (gdbscm_value_to_string): Update.
	* c-lang.c (c_get_string): Change type of "buffer".