lib/: set O_CLOEXEC on all fds

As found by
  git grep -E '(open|setmntent|pipe2?)\(' |
    grep -vE '((zfs|zpool)_|fd|dl|lzc_re|pidfile_|g_)open\('

FreeBSD's pidfile_open() says nothing about the flags of the files it
opens, but we can't do anything about it anyway; the implementation does
open all files with O_CLOEXEC

Consider this output with zpool.d/media appended with
"pid=$$; (ls -l /proc/$pid/fd > /dev/tty)":
  $ /sbin/zpool iostat -vc media
  lrwx------ 0 -> /dev/pts/0
  l-wx------ 1 -> 'pipe:[3278500]'
  l-wx------ 2 -> /dev/null
  lrwx------ 3 -> /dev/zfs
  lr-x------ 4 -> /proc/31895/mounts
  lrwx------ 5 -> /dev/zfs
  lr-x------ 10 -> /usr/lib/zfs-linux/zpool.d/media
vs
  $ ./zpool iostat -vc vendor,upath,iostat,media
  lrwx------ 0 -> /dev/pts/0
  l-wx------ 1 -> 'pipe:[3279887]'
  l-wx------ 2 -> /dev/null
  lr-x------ 10 -> /usr/lib/zfs-linux/zpool.d/media

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11866

lib/libzutil/os/freebsd/zutil_import_os.c

@@ -127,7 +127,7 @@ zpool_open_func(void *arg)
 	/*
 	 * O_NONBLOCK so we don't hang trying to open things like serial ports.
 	 */
-	if ((fd = open(rn->rn_name, O_RDONLY|O_NONBLOCK)) < 0)
+	if ((fd = open(rn->rn_name, O_RDONLY|O_NONBLOCK|O_CLOEXEC)) < 0)
 		return;
 
 	/*

lib/libzutil/os/linux/zutil_device_path_os.c

@@ -390,7 +390,7 @@ zfs_dev_is_whole_disk(const char *dev_name)
 	struct dk_gpt *label;
 	int fd;
 
-	if ((fd = open(dev_name, O_RDONLY | O_DIRECT)) < 0)
+	if ((fd = open(dev_name, O_RDONLY | O_DIRECT | O_CLOEXEC)) < 0)
 		return (B_FALSE);
 
 	if (efi_alloc_and_init(fd, EFI_NUMPAR, &label) != 0) {

lib/libzutil/os/linux/zutil_import_os.c

@@ -136,9 +136,9 @@ zpool_open_func(void *arg)
 	 * cache which may be stale for multipath devices.  An EINVAL errno
 	 * indicates O_DIRECT is unsupported so fallback to just O_RDONLY.
 	 */
-	fd = open(rn->rn_name, O_RDONLY | O_DIRECT);
+	fd = open(rn->rn_name, O_RDONLY | O_DIRECT | O_CLOEXEC);
 	if ((fd < 0) && (errno == EINVAL))
-		fd = open(rn->rn_name, O_RDONLY);
+		fd = open(rn->rn_name, O_RDONLY | O_CLOEXEC);
 	if ((fd < 0) && (errno == EACCES))
 		hdl->lpc_open_access_error = B_TRUE;
 	if (fd < 0)

lib/libzutil/zutil_import.c

@@ -1345,7 +1345,8 @@ zpool_find_import_impl(libpc_handle_t *hdl, importargs_t *iarg,
 				 * would prevent a zdb -e of active pools with
 				 * no cachefile.
 				 */
-				fd = open(slice->rn_name, O_RDONLY | O_EXCL);
+				fd = open(slice->rn_name,
+				    O_RDONLY | O_EXCL | O_CLOEXEC);
 				if (fd >= 0 || iarg->can_be_active) {
 					if (fd >= 0)
 						close(fd);
@@ -1437,7 +1438,7 @@ zpool_find_import_cached(libpc_handle_t *hdl, importargs_t *iarg)
 
 	verify(iarg->poolname == NULL || iarg->guid == 0);
 
-	if ((fd = open(iarg->cachefile, O_RDONLY)) < 0) {
+	if ((fd = open(iarg->cachefile, O_RDONLY | O_CLOEXEC)) < 0) {
 		zutil_error_aux(hdl, "%s", strerror(errno));
 		(void) zutil_error(hdl, EZFS_BADCACHE,
 		    dgettext(TEXT_DOMAIN, "failed to open cache file"));