Use directory in /tmp for temporary files

Using a standard directory for temporaries buys us performance when the
user wants it (has configured tmpfs for /tmp) and configurability
(accepts TMPDIR=).

Moving /pack to a temporary directory gives safe concurrent remote
repacks.
Simplify by replacing echo_kill with a pipefail function
Remove deprecated config gcrypt.keyring (use gcrypt.participants or default)
Remove deprecated gcrypt::ssh:// (use rsync instead, previously autofixed)

We need to remove this since git handles ssh:// URLs already. With this
change, we now use the git backend for these.
Remove deprecated /G.XxX.. repository "url fragments" (previously autofixed)
README: Don't mention keyring files, it's not relevant anymore