libpcp/z85.c:148 free(z85) leads to coredump on aix sometimes

Bug: pcp_z85_decode() segfaults at z85.c:83 if input consists of "-----" only.

Bug: pcp_z85_decode() fails if after end marker follows something, even whitespaces

key++: normalize id and lc()



allow signing using an alternate secret key, like in pcpdecrypt()

support export/import from/to pbp

re-add detached signatures, calculated with the pbp scheme but with
separate files (as I had it before).