Use sensible domain name (the DNS one) when guessing ident information

Use sensible domain name (the DNS one) when guessing ident information

Currently, the code would use getdomainname() call, which however returns
something usually unset and not necessarily related at all to the DNS
domain name (it seems to be mostly some scary NIS/YP thing).

This patch changes the code to actually use the DNS domain name, which is
also what tends to be used in emails, and we aim at emails with our ident
code.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>