Rename project to umdns

Using mdns name is a bit confusing as it's really close to Apple's mdnsd
binary name from their mDNSResponder project.
To make things worse OpenWrt's project was hosted in the mdnsd.git repo.

Rename this project to umdns which follows other projects naming schema
and is unique enough.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: John Crispin <john@phrozen.org>