Switch from PTY to pipe for service log redirection

The PTY approach caused isatty() to return true for services using
the log directive, triggering programs like fprintd (using glib) to
emit ANSI escape codes and other TTY-specific formatting in syslog.

Using a standard pipe ensures isatty() correctly returns false, so
programs produce plain text output suitable for logging.

For services that require line-buffered output, users can wrap the
command with `stdbuf -oL` as documented in doc/config/logging.md.

Fixes #455

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/config/logging.md

@@ -39,3 +39,25 @@ Log rotation is controlled using the global `log` setting.
 **Example:**
 
     service log:prio:user.warn,tag:ntpd /sbin/ntpd pool.ntp.org -- NTP daemon
+
+Output Buffering
+----------------
+
+When using the `log` directive, Finit redirects the service's stdout and
+stderr to a pipe connected to a logger process.  Programs detect this as
+non-interactive output (i.e., `isatty()` returns false) and typically
+switch from line-buffered to fully-buffered mode.
+
+Most well-behaved daemons explicitly flush their output or use syslog
+directly, so this is rarely an issue.  However, if a service's log
+messages appear delayed or batched, you can force line-buffered output
+by wrapping the command with `stdbuf`:
+
+    service log /usr/bin/stdbuf -oL /path/to/command -- My service
+
+The `-oL` option forces line-buffered output, and `-o0` forces unbuffered
+output.  See `stdbuf(1)` for details.
+
+> [!NOTE]
+> Using `stdbuf` is rarely necessary. Only use it if you observe actual
+> buffering issues with a specific service.