libfdisk: (dos) rewrite fist/last free sector functions

libfdisk: (dos) rewrite fist/last free sector functions

The current code uses first[] and last[] arrays to specify partition
ranges. This is unnecessary as we already have all in memory.

The current code offers in first and last sector dialogs ranges
without care about already used areas.

This commit makes things more readable, more user-friendly and
remove obscure first[] and last[].

Reported-by: 冰柯 <ziming_cool@126.com
Signed-off-by: Karel Zak <kzak@redhat.com>