MFC r307003, r307564: makewhatis: make output reproducible

r307003: Instead:

1. provide fts_open() with a comparison function to process directories and files in a deterministic order
2. in addition to the existing hash, insert pages into a linked list which will be sorted (by virtue of 1)
3. iterate over pages by the list in 2, instead of hash order

Idea from: des

r307564: makewhatis: avoid skipping another page after one with no mlinks

Submitted by: Ingo Schwarze

Sponsored by: The FreeBSD Foundation