Introduce BUP_DEBUG, --debug, and tone down the log messages a lot.

Introduce BUP_DEBUG, --debug, and tone down the log messages a lot.

There's a new global bup option, --debug (-D) that increments BUP_DEBUG.  If
BUP_DEBUG >=1, debug1() prints; if >= 2, debug2() prints.

We change a bunch of formerly-always-printing log() messages to debug1 or
debug2, so now a typical bup session should be a lot less noisy.

This affects midx in particular, which was *way* too noisy now that 'bup
save' and 'bup server' were running it automatically every now and then.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>