Daemonization helper
====================

When programs run as daemons, they frequently forget to detach themselves
from the parent environment. LibUCW therefore offers a simple daemonization
helper which performs the necessary actions. Namely:

* Establishing a new session via a call to `setsid()`.
* Switching the user and group ID if needed. The user and group can be specified
  by either a name or `#uid`. If only the user is given, the relevant groups are
  set according to `/etc/passwd` and `/etc/group`.
* Redirecting standard input and output from `/dev/null`. Standard error
  output is left open, so that error messages can be printed before you
  set up proper <<log:,logging>>.
* Setting the `umask()` to a fixed value (022).
* Switching from the current directory to `/`, so that it is not kept busy.
* Writing a PID file. While the daemon is running, the PID file is kept locked
  by `flock()`, so we can easily detect stale PID files.

!!ucw/daemon.h