libxbps: some random bugfixes.

* libxbps: while running INSTALL/REMOVE scripts, do not fail if a script
  returns ENOENT but the script exists; that means the script did not match
  the expected action on its switch statement and the fact can be simply
  ignored.

* libxbps: while upgrading packages and searching for obsolete files,
  do not remove the files that did not match previous SHA256 hash. Also,
  only remove obsolete dangling symlinks.

lib/unpack.c

@@ -183,9 +183,9 @@ unpack_archive_fini(struct archive *ar, prop_dictionary_t pkg)
 				}
 			}
 			
-			if ((rv = xbps_file_chdir_exec(rootdir, buf, "pre",
-			     pkgname, version, update ? "yes" : "no",
-			     NULL)) != 0) {
+			rv = xbps_file_chdir_exec(rootdir, buf, "pre",
+			     pkgname, version, update ? "yes" : "no", NULL);
+			if (rv != 0 && errno != ENOENT) {
 				free(buf);
 				fprintf(stderr,
 				    "%s: preinst action target error %s\n",