Prefer < 0 to == -1 where either will do

Also, fix an unlikely read overflow in sys_exec_setmtime_script.
* src/buffer.c (open_compressed_archive):
* src/compare.c (verify_volume):
* src/exclist.c (info_attach_exclist):
* src/misc.c (xfork):
* src/sparse.c (sparse_scan_file_seek):
* src/system.c (sys_wait_for_child, sys_spawn_shell)
(wait_for_grandchild, sys_wait_command, sys_exec_info_script)
(sys_exec_checkpoint_script, sys_exec_setmtime_script):
* src/transform.c (_single_transform_name_to_obstack):
* src/xattrs.c (xattrs__acls_set, xattrs_acls_get)
(xattrs_xattrs_get, xattrs__fd_set, xattrs_selinux_get)
(xattrs_selinux_set):
* tests/checkseekhole.c (check_seek_hole, main):
Simplify failure tests by just looking at return value sign.
* src/system.c (sys_exec_setmtime_script):
Don’t assume ‘read’ result fits in int.
(sys_exec_setmtime_script): Don’t reject 1 second before Epoch.

src/transform.c

@@ -508,8 +508,8 @@ _single_transform_name_to_obstack (struct transform *tf, char *input)
 		  break;
 
 		case segm_backref:    /* Back-reference segment */
-		  if (rmp[segm->v.ref].rm_so != -1
-		      && rmp[segm->v.ref].rm_eo != -1)
+		  if (0 <= rmp[segm->v.ref].rm_so
+		      && 0 <= rmp[segm->v.ref].rm_eo)
 		    {
 		      size_t size = rmp[segm->v.ref].rm_eo
 			              - rmp[segm->v.ref].rm_so;