Return unique_xmalloc_ptr from target_fileio_read_stralloc

Return unique_xmalloc_ptr from target_fileio_read_stralloc

Change target_fileio_read_stralloc to return unique_xmalloc_ptr and
fix up the callers.  This removes a number of cleanups.

ChangeLog
2017-10-16  Tom Tromey  <tom@tromey.com>

	* linux-tdep.c (linux_info_proc, linux_find_memory_regions_full)
	(linux_fill_prpsinfo, linux_vsyscall_range_raw): Update.
	* target.c (target_fileio_read_stralloc): Update.
	* sparc64-tdep.c (adi_is_addr_mapped): Update.
	* target.h (target_fileio_read_stralloc): Return
	unique_xmalloc_ptr.