libblkid: add blkid_probe_all_removable()

libblkid: add blkid_probe_all_removable()

The libblkid probing is based on devices from /proc/partitions by
default. This file usually does not contain removable devices (e.g.
CDROMs) and this kind of devices are invisible for libblkid.

The blkid_probe_all_removable() function adds removable block devices
to blkid cache. The probing is based on information from the /sys
directory. The devices which were detected by this function won't be
written to blkid.tab cache file.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=533874
Signed-off-by: Karel Zak <kzak@redhat.com>