Extract the larger bsd.port.mk targets into separate scripts.

Refactor all the fetch code so that there are not 6 slightly different
versions of it but one that does it all.

The targets that have been extracted are:

• check-vulnerable
• do-fetch
• fetch-list
• fetch-url-list-int
• fetch-urlall-list
• checksum.
• makesum.
• check-checksum-algorithms

Run the fetch code directly from make makesum instead of calling make
fetch, this is because some port change the options with OPTIONS_*_FORCE
when make(makesum) to be able to add all distfiles in one go, which was
a nice, non working, idea.

PR: 208916
Submitted by: mat
Exp-run by: antoine
With hat: portmgr
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D5997