FIX: enhanced nbdkit command detection and usage

[414n]

On some Linux distributions (like Slackware Linux), directories like /sbin and /usr/sbin are not included in the default PATH for regular users. As a result, attempting to launch nbdkit as is done in lib/nbdkit.ml fails at runtime because the binary is typically installed in /usr/sbin.

This PR does the following:

1. detect nbdkit earlier at configure time in guestfs-progs.m4, by explicitly searching it also in /usr/sbin and /sbin. Failure to detect it results in a warning message in order to keep it as an optional dependency;
2. inject the detected path in the Config module, but keeping it an option type;
3. refactor lib/nbdkit.ml to use the detected path instead of the hardcoded nbdkit.