Document some user-settable make variables in ports.7

This is not a comprehensive list, as the variables themselves are spread
out over multiple files, but it is a start.

Add a section to make.conf noting that variables may be set there that
affect ports builds, but refer to ports.7 and elsewhere for the actual
listing; any listing in make.conf.5 would likely become out of date
fairly quickly.

PR:		docs/189199
Reviewed by:	bdrewery (previous version)
Approved by:	hrs (mentor)
Assorted updates to md5.1

Note that the -c argument's parameter is compared against the digest of
the file, not the file. [1]

Update the "current time" parentheticals for notes about reversing
and colliding the hash functions. [1]

Some general mdoc updates.

PR:		docs/188043 [1]
Submitted by:	Jamie Landeg-Jones [1]
Approved by:	hrs (mentor)
MFC after:	1 week