This is for helper scripts and programs that I don't want on PATH.
Shell pipelines, scripts for `git bisect`, and Perl one-liners turned
into files commencing `#!/usr/bin/perl -wnl`, are the sort of thing we
have in here.

Loosely categorise the scripts, e.g. dgit/ might contain tools useful
when developing dgit.

Note that repos other than ~/src/dotfiles might stow into ~/lib/aid.