Do chroot always.
Absolutize the config file name if it's not.

(Instead of earlier conditional chrooting.)
Add config_save_root option.
Save config under config_save_root.
Use config_to_buffer in config_dump.
Log if we got interrupted by a signal.
Add lock_file option.
There can be only one.

On startup, block on lock file; on shutdown, unlock it.
The lock file contains the pid of the locking process.
Revert "Restore the spill root built-in default to /tmp, less hassle."

This reverts commit 5712bf705a5cdf66d8a7073180007fb89c9b1a1f.