MFC r298901:
restore: promote some getfiles() parameters to size_t.

This is based on a change from OpenBSD:

"Fix restore so that it can actually restore files larger than 4GB by
changing the type of "size" to off_t in getfiles() plus little dependent
type cleanup, from Daniel Lucq."

It is an important for machines with 32 bit longs.
While here unsign the flags, also from OpenBSD.

Obtained from: OpenBSD (with changes)