MFC r365490-r365491: Remove FREEBSD_UPDATE dependency on PORTSNAP

MFC r365490-r365491: Remove FREEBSD_UPDATE dependency on PORTSNAP

r365490:
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.

r365491:
opts: FREEBSD_UPDATE no longer relies on PORTSNAP

phttpget is no longer tied to the portsnap build as of r365490.