libparted: make pc98 detection depend on signatures

libparted: make pc98 detection depend on signatures

pc98 is not a common disk label. Change pc98_probe to only return true
if one of the recognized signatures is present.
Currently these include:

IPL1
Linux 98
GRUB/98

This prevents false-positive detection on msdos labeled disks.

* libparted/labels/pc98.c (pc98_probe): Change to require signature
(pc98_check_ipl_signature): Add more signatures
(check_partition_consistency): Remove unused function
Reported by Zach Carter in http://bugzilla.redhat.com/646053