Fix salt.modules.mount for FreeBSD

I've changed the way in which the mount.active function worked to use the `mount -p` command on FreeBSD as it doesn't support either of the `/proc/self` methods currently implemented.

I've also changed the way in which the `mount` arguments in mount and remount functions are created to ensure that flags/switches are passed first before the plain arguments as this was causing an invocation error when calling `mount` on FreeBSD.

The argument changes should be fine to pull, the mount.active I'm less sure about. I don't know if its better to look at the grains 'os' or to add another try/except block but I'm not a fan of having them too deeply nested.