Many code improvements

The following improvements come from Guilhem Moulin <guilhem@guilhem.org>

    keyart [-f FILE] [KEYID [KEYID ...]]

    Each keyring or key file now has to be prefixed with -f.  If no KEYID is
    provided, display an ASCII art representation of all keys found in said
    ---
    1. Make only one call to gpg, regardless of the number of key IDs
       provided.

    2. Use 'gpg --with-colons --fingerprint --no-default-keyring
      --keyring=/path/to/keyring [--keyring=/path/to/keyring2 ...]' contruction,
      which is immune to GnuPG bug #1640.
    ---
    prevent key files and key IDs to be interpreted as gpg options.
    ---
    Enable key identifiers that identify multiple keys.