Backport upstream fix for OCaml 4 to avoid SIGSEGV

Unison 2.32.x consistently crashes on startup after reading its configuration
file with a SIGSEGV and does not serve its purpose at all.

Pull in upstream fix from
https://github.com/bcpierce00/unison/commit/891c081690ae4605c9bf818ee89fbdf2416b3dd7#diff-4117bab3eb479a1766c95c36bb61dd35c817a76e57f9c4c9a430948f504453de

Suggested by madpilot@ in https://reviews.freebsd.org/D26924#600631

While here, run the selftest as part of the build to catch such issues
in the future and receive pkg-fallout@ mail.

While here, also remove MAKE_JOBS_UNSAFE.

Bump PORTREVISION to trigger a rebuild and flush out the broken packages.

(The absence of bug reports however suggests that unison 2.32 is no longer
used or needed widely.)

Obtained from: Benjamin Pierce's upstream repo (madpilot@)
MFH: 2020Q4 (blanket, unbreak program, single patch)