Silence warning printed by getfsspec(3) when /etc/fstab does not exist

fstab: /etc/fstab:0: No such file or directory
and from dump(8) when setfsent(3) fails due to /etc/fstab not existing:
  DUMP: Can't open /etc/fstab for dump table information: No such...

This makes daily and security periodic runs somewhat cleaner in jails
which lack /etc/fstab files.

MFC after:	1 month

etc/periodic/daily/400.status-disks

@@ -19,6 +19,9 @@ case "$daily_status_disks_enable" in
 	df $daily_status_disks_df_flags && rc=1 || rc=3
 
 	# display which filesystems need backing up
+	if ! [ -f /etc/fstab ]; then
+		export PATH_FSTAB=/dev/null
+	fi
 
 	echo ""
 	dump W || rc=3;;

etc/periodic/security/200.chkmounts

@@ -50,6 +50,9 @@ case "$daily_status_security_chkmounts_enable" in
 		ignore="${ignore}|^amd:"
 	esac
 	[ -n "$ignore" ] && cmd="egrep -v ${ignore#|}" || cmd=cat
+	if ! [ -f /etc/fstab ]; then
+		export PATH_FSTAB=/dev/null
+	fi
 	mount -p | sort | ${cmd} |
 	  check_diff mount - "${host} changes in mounted filesystems:"
 	rc=$?;;