WIP: Add src/psk2.c using new src/options.c [skip ci]

PoC to replace libopts dependency. src/psk2.c is a copy of src/psk.c which uses the new src/options.c. doc/manpages/Makefile.am generates man, texi, info from psk2tool and psk2tool.md.in (a markdown template).

Can we replace pandoc by something else with less dependencies (e.g. help2man) ?

Issue for discussion is #775 (closed)