debian: Use machine-readable debian/copyright file.

debian: Use machine-readable debian/copyright file.

Machine-readable copyright files are generally recommended to deal
with the diversity of free software licenses. See

	https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/

for details. Since the package contains epigrams in the public domain
as well as source code which is licensed under the GPL-3, we need
two Files stanzas and two stand-alone License stanzas, plus the
required header stanza.

By convention, the last matching Files: pattern applies, so the pattern
for the epigrams needs to listed *after* the asterisk pattern that
matches everything.

Suggested-by: lintian(1)
Reviewed-by: Andreas Metzler <ametzler@bebt.de>