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()

use recipient in encryption to lookup public key in the vault, if id not given

change encrypted file format to the one of pbp, following it:
   - support multiple recipients
   - encrypt 32k blockwise using crypto_secretbox() using
     a random key, encrypt that key for each recipient with
     pk using crypto_box()
   - base85 <=> z85? maybe make us base85 tolerant while still
     preferring z85? I dunno...

support export/import from/to pbp

remove key-id from stored signatures, maybe add the file content again
(by default a signature consists of the file+sig, but I sometime decided
to detach the sig and only write this to the signature file, at least pbp
stores the full stuff, so I shall do it as well).