libxbps: use OpenSSL SHA256, 5x faster xbps_get_file_hash() implementation.

* libxbps: improved xbps_get_file_hash() to mmap(2) rather than read(2)
  the file we have to process. With mmap'ed files the hash can be
  processed 5x faster than before, or even more in some cases.

* libxbps: switch to OpenSSL SHA256 implementation and remove the one
  previously used. It's faster and OpenSSL is required for libfetch so
  there is not point in using it.

lib/Makefile

@@ -36,7 +36,7 @@ OBJS = configure.o config_files.o cmpver.o download.o fexec.o
 OBJS += humanize_number.o orphans.o plist.o purge.o register.o
 OBJS += regpkgs_dictionary.o remove.o remove_obsoletes.o repository.o
 OBJS += repository_finddeps.o repository_findpkg.o repository_plist.o
-OBJS += repository_pool.o repository_sync_index.o requiredby.o sha256.o
+OBJS += repository_pool.o repository_sync_index.o requiredby.o
 OBJS += sortdeps.o state.o unpack.o util.o pkgmatch.o mkpath.o
 OBJS += $(COMPAT_SRCS)