[new] security/rnp 0.15.2

Hi,

here's a port of rnp (cf https://www.rnpgp.org/ and
https://www.rnpgp.org/software/rnp/) that powers the pgp
implementation used by thunderbird - someday (cf
https://bugzilla.mozilla.org/show_bug.cgi?id=1732809) the latter will
support --with-system-rnp, so might aswell have the standalone lib in
ports instead of building the bundled one as of now.

all tests pass fine (it depends on gpg for interop testing ofc), COMMENT
and DESCR could use improvements, other than that feedback welcome.

Landry