mailsplit and mailinfo: gracefully handle NUL characters

mailsplit and mailinfo: gracefully handle NUL characters

The function fgets() has a big problem with NUL characters: it reads
them, but nobody will know if the NUL comes from the file stream, or
was appended at the end of the line.

So implement a custom read_line_with_nul() function.

Noticed by Tommy Thorn.

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