Update rc.initdiskless, fix error handling of remount_optional

chkerr() ignores the exit code of a preceding mount command in case a
file ```remount_optional``` exists.  The check is performed and a
subshell is launched to log the informational message and return.  The
return is executed in the context of the subshell, not the context of
the chkerr() function, hence is a NOP.  The remount_optional check is
hence ineffective.

Change the code to if/then/fi, so the return is evaluated in the context
of the chkerr function, to make the check effective.

Reviewed by: imp, emaste
Pull Request: https://github.com/freebsd/freebsd-src/pull/1497

libexec/rc/rc.initdiskless

@@ -174,7 +174,10 @@ log() {
 chkerr() {
     lastitem () ( n=$(($# - 1)) ; shift $n ; echo $1 )
     mountpoint="$(lastitem $2)"
-    [ -r $mountpoint/remount_optional ] && ( echo "$2 failed: ignoring due to remount_optional" ; return )
+    if [ -r $mountpoint/remount_optional ]; then
+        echo "$2 failed: ignoring due to remount_optional"
+        return
+    fi
     case $1 in
     0)
 	;;