x/ref/lib/cmdline: improve copyright handling in gendoc
x/ref/lib/cmdline: improve copyright handling in gendoc

gendoc currently always includes the Vanadium copyright message. This
change adds a flag to specify an arbitrary copyright message instead. If
the flag, --copyright-notice, is set to an empty string then no
copyright message is included, otherwise it refers to a file containing
the message to be used.
vanadium/go.lib