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()

malloc() new pointers in functions only if not NULL, e.g. pcp_gennonce()

generalize file i/0, open+close only in src/, print msg if using stdin or stdout

put the key import and export stuff into the lib, support from/to file and string

check pub key count in pcp.c before calling verify signature, croak if count==0

fix encrypt+sign protocol (http://world.std.com/~dtd/sign_encrypt/sign_encrypt7.html),
by: hash the message, at the end, sign that hash, then encrypt the result, append to
cipher output. include recipient list also in the signature.