Remove remnants of portsnap(8)

This was prompted by noticing that '/var/db/portsnap' still exists on
newly-installed machines.

With this change, all mentions of portsnap(8) in the tree are gone,
except for the historical note in the AUTHORS section of manpage
phttpget(8).

locate(1) will thus start indexing again '/var/db/portsnap' on machines
where this directory still exists, which may be a good way to push
administrators to delete it.

Reviewed by: cperciva
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45023

Remove $FreeBSD$: two-line nroff pattern

Remove /^\.\\"\n\.\\"\s*\$FreeBSD\$$\n/

phttpget: move out of portsnap

Currently, WITHOUT_PORTSNAP forces WITHOUT_FREEBSD_UPDATE because the
latter relies on phttpget, which lives inside the portsnap build bits.

Remove the dependency between these two options by moving phttpget out into
^/libexec and building/installing it if either WITH_PORTSNAP or
WITH_FREEBSD_UPDATE.

Future work could remove the conditional if it's decided that users will use
it independently of either the current in-base consumers.

Reported by: swills
Reviewed by: jilles, emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D26255