MFC r290024,290073:

In gunzip(1), treat trailing garbage as a warning and not an error. This
allows scripts to distinguish it between real fatal errors, for instance a
CRC mismatch.

Update manual page for the behavior change.

PR: bin/203873
Submitted by: Eugene Grosbein <eugen grosbein net>

MFC r281500,281540,281626:

Sync with NetBSD:
- Mention xz(1) in gzip(1).
- Strip away path from header name when decompressing.

MFC r273507: Sync with NetBSD.

MFC r281500,281540,281626:

Sync with NetBSD:
- Mention xz(1) in gzip(1).
- Strip away path from header name when decompressing.

MFC r273507: Sync with NetBSD.