kbuild, deb-pkg: improve maintainer identification

Try harder to find email and maintainer name.
Debian's own devscripts all use DEBEMAIL or DEBFULLNAME prior to an
eventual EMAIL or NAME environment variable. Match their logic.

"Anonymous" sounds nicer then "Kernel Compiler" if no name is found.

Signed-off-by: maximilian attems <max@stro.at>
Signed-off-by: Frans Pop <elendil@planet.nl>
Cc: Andres Salomon <dilinger@debian.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>