Point people to the the-tcpdump-group repositories, not the mcr repositories.

We've created a the-tcpdump-group organization on GitHub, and created
repositories for libpcap and tcpdump, owned by them.  Those are now the
"official" GitHub locations for repositories from which to clone or
against which to file issues/pull requests.
For "...by forking the branch", give the right URLs.

Give the URLs that will take you to the page on the GitHub Web site for
the trunk of the libpcap or tcpdump repository; presumably the idea is
that the user will go there and do the fork and issue the pull request
from their browser.
Give instructions on how to clone the GitHub repositories.

Fix more links, and give the "git clone" commands to use when cloning
the GitHub repositories rather than the bpf.tcpdump.org repositories.

Also fix some lines with embedded CRs.
1.4.0/4.4.0 release
Merge branch 'master' of git+ssh://bpf.tcpdump.org/tcpdump/master/git/htdocs
newly signed TCPDUMP 4.4 release
note about new TCPDUMP signing key
Merge remote-tracking branch 'github/master'
Add LINKTYPE_BLUETOOTH_LE_LL/DLT_BLUETOOTH_LE_LL.

Also fix a formatting error.
Get rid of the RTAC serial linktype for now.

Some discussions are still occurring on it; we'll add it back when we
finish.
Put LINKTYPE_RTAC_SERIAL in.
Merge branch 'master' of git+ssh://bpf.tcpdump.org/tcpdump/master/git/htdocs

This merge is necessary because Git appears to think it's necessary.  I
guess I forgot to do a "git pull" just before my push.
LINKTYPE_BLUETOOTH_LE_LL doesn't include the preamble.
Note that you have to check the link-layer header type.

(Somebody on Stack Overflow got confused by this; they tried using the
sample code, parsing Ethernet headers, on the OS X loopback device.)
Merge remote-tracking branch 'mcr/master'