Install from OpenBSD packages

We are now in the OpenBSD ports tree, so add pkg_add(1) instructions.
This only works on -current right now.

While here, re-arrange installation instructions to be alphabetical by
OS (or distribution) name.
Manually resolve symlinks in test helpers

Mac OS X's `readlink` command does not support the same options as GNU,
FreeBSD, and other operating systems, nor does it support canonicalizing
symlink resolutions.

In place of `readlink`, we're going to borrow [this] suggested
implementation from the community.

[this]:
http://stackoverflow.com/questions/1055671/how-can-i-get-the-behavior-of-gnus-readlink-f-on-a-mac
Fix broken mailmap documentation link

It was not the full path to the docs, but let's also just refer to the version
on master.