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.