Convert CR/LF to LF in tag signatures

Convert CR/LF to LF in tag signatures

On Windows, gpg outputs CR/LF signatures.  But since the tag messages
are already stripped of the CR by stripspace(), it is arguably nicer
to do the same for the tag signature.  Actually, this patch does not
look for CR/LF, but strips all CRs from the signature.  It does so not
only on Windows but on all platforms to keep the code simpler.

[ spr: ported code to use strbuf ]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>