Adopt BusyBox init signal handling for halt/reboot/poweroff

This patch adopts the BusyBox init signals for handling halt, reboot,
and poweroff.  The SIGINT and SIGPWR signals still behave the same as
before, even though they likely will become configurable in a future
release.

Note: the disabling of Ctrl-Alt-Delete (SIGINT) is also removed!

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

README.md

@@ -245,12 +245,45 @@ start any new ones for you, without rebooting the system.
 Rebooting & Halting
 -------------------
 
-Finit handles `SIGUSR1` and `SIGUSR2` for reboot and halt, and listens
-to `/dev/initctl` so system reboot and halt commands also work.  The
-latter is implemented in the optional `initctl.so` plugin and can be
-accessed with the traditional `telinit` command line tool, symlinked to
-`finit`.  Hence, if finit is your system init, then `init q` will work
-as the UNIX beards intended.
+Finit is often used on embedded and small Linux systems with BusyBox.
+For compatibility with its existing toolset (halt, poweroff, reboot),
+the following signals have been adopted:
+
+* `SIGUSR1`  
+  Same effect as `finit q`, reloads all *.conf files in `/etc/finit.d/`
+* `SIGUSR1`  
+  Calls shutdown hooks, including HOOK_SHUTDOWN, stops all running
+  processes, and unmounts all file systems.  Then tells kernel to
+  halt.  Most people these days want SIGUSR2 though.
+
+  SysV init and systemd use this to re-open their FIFO/D-Bus.
+* `SIGUSR2`  
+  Like SIGUSR1, but tell kernel to power-off the system, if ACPI
+  or similar exists to actually do this.  If the kernel fails
+  power-off, Finit falls back to halt.
+ 
+  SysV init N/A, systemd dumps its internal state to log.
+* `SIGTERM`  
+  Like `SIGUSR1`, but tell kernel to reboot the system when done.
+ 
+  SysV init N/A, systemd rexecutes itself.
+* `SIGINT`  
+  Sent from kernel when the CTRL-ALT-DEL key combo is pressed.
+  SysV init and systemd default to reboot with `shutdown -r`.  
+  Finit currently forwards this to `SIGTERM`.
+* `SIGPWR`  
+  Sent from a power daemon, like `powstatd(8)`, on changes to the
+  UPS status.  Traditionally SysV init read /etc/powerstatus and
+  acted on "OK", "FAIL", or "LOW" and then removed the file.  
+  Finit currently forwards this to `SIGUSR2`.
+
+Finit also listens to the classic SysV init FIFO, which used to be
+located in `/dev` but these days default to `/run/initctl`.  So existing
+tools like, `poweroff`, `halt`, `reboot`, and `shutdown` work as
+expected.  Support for this old-style handling is implemented in the
+optional `initctl.so` plugin and can be accessed with the traditional
+`telinit` command line tool, symlinked to `finit`.  Hence, if finit is
+your system init, then `init q` will work as the UNIX beards intended.
 
 ```shell
     ~ $ telinit