bb_full_fd_action: remove potential xmalloc from NOFORK path

cat: stop using stdio.h opens
libbb: introduce & use open[3]_or_warn
function                                             old     new   delta
open3_or_warn                                          -      54     +54
bb_cat                                               115     144     +29
open_or_warn                                           -      25     +25
unlzma                                              2404    2412      +8
chattr_main                                          334     339      +5
xstrtoul_range_sfx                                   251     255      +4
telnet_main                                         1514    1510      -4
static.opt                                             4       -      -4
qgravechar                                           122     118      -4
fuser_add_pid                                         61      54      -7
fuser_add_inode                                      154     147      -7
writeFileToTarball                                  1542    1534      -8
refresh                                             1156    1148      -8
do_show                                              856     846     -10
read_leases                                          212     200     -12
setup_redirects                                      236     222     -14
iproute_list_or_flush                               1582    1568     -14
read_config                                          427     411     -16
write_leases                                         284     264     -20
hash_file                                            338     318     -20
copy_file                                           1760    1740     -20
do_iproute                                          2610    2588     -22
bb_full_fd_action                                    320     269     -51
open_to_or_warn                                      103      49     -54
fuser_main                                          1660    1596     -64
.rodata                                           131160  131096     -64
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 4/19 up/down: 125/-423)        Total: -298 bytes

1 files changed, 4 insertions, 6 deletions

diff --git a/libbb/copy_file.c b/libbb/copy_file.c
index 7d85920d9..700564212 100644
--- a/libbb/copy_file.c
+++ b/libbb/copy_file.c
@@ -217,9 +217,8 @@ int copy_file(const char *source, const char *dest, int flags)
 			add_to_ino_dev_hashtable(&source_stat, dest);
 		}
 
-		src_fd = open(source, O_RDONLY);
-		if (src_fd == -1) {
-			bb_perror_msg("cannot open '%s'", source);
+		src_fd = open_or_warn(source, O_RDONLY);
+		if (src_fd < 0) {
 			return -1;
 		}
 
@@ -237,9 +236,8 @@ int copy_file(const char *source, const char *dest, int flags)
 				return ovr;
 			}
 			/* It shouldn't exist. If it exists, do not open (symlink attack?) */
-			dst_fd = open(dest, O_WRONLY|O_CREAT|O_EXCL, source_stat.st_mode);
-			if (dst_fd == -1) {
-				bb_perror_msg("cannot open '%s'", dest);
+			dst_fd = open3_or_warn(dest, O_WRONLY|O_CREAT|O_EXCL, source_stat.st_mode);
+			if (dst_fd < 0) {
 				close(src_fd);
 				return -1;
 			}