murphy-db: don't propagate uninitialized value to status.
config: don't try to load D-Bus plugin.
daemon: make sure stdout is line-buffered (and stderr non-buffered).

This should fix the annoying "no output from console commands" bug
experienced when running/logging via systemd. When running under
systemd fileno stdout is a socket. Hence, stdout is fully buffered
by default with a quite large buffer size. This effectively breaks
the interactive murphy console (unless we fflush both stdout and
stderr after running a command handler) as output resulting from
a console command gets buffered and is only written to the console
at random intervals when the buffer gets full.
console: flush and proxy output after each command.

Also we have now an I/O watch in place for the console
output pipe. This enables us in the future to plug any output fd
to any console if we wish to do so.
resolver: filter out duplicate target dependencies.

Currently failing to do so could trigger dereferencing
fact_stamps beyond its end in older_than_facts (as
fact_stamps is allocated in the graph sorter and its
size reflects the number of unique dependencies).
common: don't free a busy nl_pid_watch_t.

Mark an nl_pid_watch_t busy while it is being scanned. If a
client callback destroys the last watch, check if the pid
watch is busy. If it is, just unhash it and mark it dead,
otherwise unhash and destroy it right away. Each time a watch
scan finishes, check and destroy the watch if it has been
marked dead.