Use rkennedy/git-extras and set up MANPATHrc

My git-extras repo includes a change that makes the man folder suitable
for direct inclusion in the `MANPATH` environment variable, so now `man`
works with all the git-extras commands without any installation.
Remove reference to PATHrc in readme

It doesn't need to be linked from the home directory anymore. We read it
directly from the repo folder via `$DOTFILES`.
More portable determination of DOTFILES

While Mac OS X doesn't support the `-f` option for `readlink`, my
Solaris environments don't support `readlink` at all. Thus, a Perl
implementation seems to be a more reliable answer here.