sysutils/e2fsprogs: fix "blkid" probe-all feature

e2fsprogs's misc/blkid utility would expect a /proc/partitions file
in a Linux-specific format. Add code to read the kern.geom.conftxt
sysctl(3) value instead, and adjust parser.

While here, patch /etc/blkid.tab location to ${ETCDIR}.

Reported by: Adam Dobrawy (private mail)
Tip to: bapt@ (for the pointer to kern.geom.conftxt)