Add runtime configuration to perl-nocem to customize settings

[Julien-Elie]

Following a suggestion of Richard Kettlewell, it would be worthwhile adding flags to perl-nocem so as to change the default parameters:

• the key ring to use (instead of the hard-coded ncmring.gpg file),
• the GnuPG binaries to use and the flags to give them,
• enabling logging to a log file,
• the name of the log file...