Re-implement $probe_only aspect of f_media_get_TYPE() (where TYPE is cdrom,
nfs, ftp, http, httpproxy, etc.) and f_device_get() (abstract method for
calling aforementioned f_media_get_TYPE()).

Previously, if $probe_only was present and non-NULL, the TYPE functions
would check for $file and exit with an appropriate error status (success if
the file exists and readable, failure otherwise).

While this has been retained, a pair of globals has been introduced:
$PROBE_EXIST and $PROBE_SIZE (see `/usr/share/bsdconfig/media/common.subr')

The $PROBE_EXIST global can be used where you need the functionality of
simply testing for existence (previously the _only_ functionality).

Meanwhile, the new $PROBE_SIZE global can be used to cause the TYPE function
to print the size of the file (in bytes) on standard-out (or -1) if not
found or an error occurs. NOTE: If an error occurs, it is logged with the
dprintf function, which is visible with `-d' flag or debug=1.

In many cases, where you need to get the size of a file _and_ check for its
existence, you can use the return status of a $PROBE_SIZE call.