maint: licensing conformance follow-up (library->program)

Once we got clarified "GPLv2.1+ -> GPLv2+" (rejecting LGPL
interpretation, cf. 7090581), we ought to reword "library"
references to say "program", in accordance to the standard
GPLv2 notice.

In the same vein, update Pod documentation to refer to the
correct version(s) of GPL.