Use sysconfdir & localstatedir for conf/pid/logs

Use autoconf-supplied paths as the default values for the configuration
file, the pidfile and the logfile. So, for example:
  /etc/nutcracker/nutcracker.yml
  /var/run/nutcracker/nutcracker.pid
  /var/log/nutcracker/nutcracker.log

The separate subdirectory under /var/run was picked since nutcracker
doesn't possess the capability of dropping privileges by itself, so we
need to have a system user writeable directory to place the pidfile.

Similarly, we need a separate /var/log subdir as logs can't go to
syslog, and out of convenience (need to be able to perform logrotate,
cleanup upon removing a package etc.)

The separate subdirectory under /etc/ is just for being pretty
(top-level /etc conffiles are ugly) and for future use (multiple configs
for the same daemon?)

All of the above can of course be used in the case where a single init
script spawns multiple nutcrackers, each with a separate config, pidfile
and logfile, for example for extra redundancy reasons.

(also fix an underlying bug where NC_PID_FILE was completely unused)