[Libguestfs] [PATCH nbdkit 0/2 PROPOSAL] log: Add logscript feature.

This patch series isn't finished - at a minimum it needs tests. As you may have seen I've been fiddling around with nbdkit-log-filter, generally tidying up the code. However the big change I want to make is to allow a script to be triggered from logging events. The two patches that follow show where this is going. The first patch cleans up the log format itself so that we consistently use shell (actually _bash_) quoting for the parameters that we print in the log file. Note this introduces some minor incompatibilities. This allows the second patch which implements the logscript feature by passing the same parameters directly to an external script. Rich.