Recognize all spellings of remote mDNS hostnames

Remote candidates with mDNS (.local) hostnames are not paired with relay
candidates, so that their resolved address isn't sent to the TURN
server, and their resolved address is concealed in stats. Both checks
compared the end of the hostname with ".local" byte by byte. However,
DNS names are case-insensitive and may be written in fully qualified
form with a trailing dot. Names such as "name.local." or "NAME.LOCAL"
are resolved using mDNS as well, so such candidates bypassed both
checks.

Add IsMdnsHostname(), which strips a trailing dot and compares the
.local suffix case-insensitively, and use it for both checks.

Bug: chromium:504576161
Change-Id: I3b3f30fbd0d10680e928a759c61755e4f0b07f57
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/507180
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48792}