cvsserver: change generation of CVS author names

cvsserver: change generation of CVS author names

CVS username is generated from local part email address.
We take the whole local part but restrict the character set to the
Portable Filename Character Set, which is used for Unix login names
according to Single Unix Specification v3.

This will obviously report different usernames from existing repositories
for commits with the local part of the author e-mail address that contains
characters outside the PFCS.  Hopefully this won't break an old CVS
checkout from an earlier version of git-cvsserver, because the names are
always shown afresh to the CVS clients and not kept on the client side.

Signed-off-by: Fabian Emmes <fabian.emmes@rwth-aachen.de>
Signed-off-by: Lars Noschinski <lars@public.noschinski.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>