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