Parse pks_bin_dir / mail_dir in the C config handling

Parse pks_bin_dir / mail_dir in the C config handling

pks_bin_dir and mail_dir are only used by the Perl that handles
incoming keyserver email, but parse them in the C code rather than
just ignoring them. This helps pave the way for some config file format
changes and tightening up the parsing to complain about unused options.