Document the installation and packaging of gitwrapper for -portable

On some systems libmd isn't explicitly implied, so detect this via pkg-config at configure time. Problem reported by @stsp

The default branch in -portable has changed from linux -> portable, and so the documentation needs updating as well.

Some code in compat/ has #defines which were not being checked for in configure.ac, which could have lead to compilation failures were those files to be included on systems which needed them. In doing so, we can now remove libmd and libcrypto as these are no longer needed.

Mention that GNU coreutils is no longer required on *BSDs to run the test suite.

When creating a release tarball, use a GOT_RELEASE variable to determine the version of the release.

When compiling got-portable from source, outside of an official release tarball, it's often helpful to know which commit the binary was built from, especially for diagnostic purposes. This adds an extra hook to AC_INIT to derive whether -portable is being built in-tree or via a release tarball.

Document how to release a new `-portable` version.

Add a section to the README files explaining how -portable is maintained.

When writing -portable patches, mention the format of the commit's shortlog.

When running tests on non-Linux systems, it was just assumed that GNU coreutils had been installed. This meant that the wrapper commands for date/ln on those systems always had gdate and gln installed. Rather than assume that, check to see if the GNU coreutils versions as present, and if not echo a warning.

This adds libbsd to the README files to indicate a core dependency. Originally from Ashish SHUKLA.

When running the tests under Linux, jot(1) can be found in the athena-jot package.

This adds support for DragonFlyBSD.

Mention which operating systems are supported.

Although configure.ac will for the presence of bison, the READMEs didn't say that was a required program. Patch from Carlos Torres

Remove conflict markers in README, and add mailing-list link to both READMEs. Noticed by stsp

Fix some typos. From Christian "naddy" Weisgerber

Add a .github/README.md file so that the read-only clone of GoT on Github, which is used for CI purposes, explains the project. Also, update the README.portable vesion to patch.