Change target_read_string to use unique_xmalloc_ptr

Change target_read_string to use unique_xmalloc_ptr

This changes the out parameter of target_read_string to be a
unique_xmalloc_ptr.  This avoids a cleanup and sets the stage for more
cleanup removals.

This patch also removes a seemingly needless alloca from
print_subexp_standard.

gdb/ChangeLog
2018-03-30  Tom Tromey  <tom@tromey.com>

	* windows-nat.c (handle_output_debug_string, handle_exception):
	Update.
	* target.h (target_read_string): Update.
	* target.c (target_read_string): Change "string" to
	unique_xmalloc_ptr.
	* solib-svr4.c (open_symbol_file_object, svr4_read_so_list):
	Update.
	* solib-frv.c (frv_current_sos): Update.
	* solib-dsbt.c (dsbt_current_sos): Update.
	* solib-darwin.c (darwin_current_sos): Update.
	* linux-thread-db.c (inferior_has_bug): Update.
	* expprint.c (print_subexp_standard) <case OP_OBJC_MSGCALL>:
	Update.  Remove alloca.
	* ada-lang.c (ada_main_name): Update.