Update bash/functions/bashmod

Not finding a module is fatal: error out from the calling script
Update bash/functions/bashmod

Use bashmod script directory as default BASH_MODULES_PATH if unset. This
allows not having to bother with environment variables in simple cases.
Update misc/check-hardlinks, admin/safeshutdown, admin/gentoo/purge-distfiles

Switch to bashmod to source required Bash modules. Use sensible defaults
for BASH_MODULES_SCRIPT to avoid having to set it in the repo
environment.
Update README.md

No more minimal setup if everything stays inside the repo.