Drop parsing of /proc/cmdline by default, instead use argc + argv[]

For most use-cases the kernel will give Finit its arguments as proper
command line args in argc + argv[], like any other program.  However,
for some users, most notably Alpine Linux, there is a slightly broken
initramfs that cannot forward more than one argument using init_args,
for such systems you can re-enable the old behavior with a configure
switch --enable-kernel-cmdline -- it's not ideal but what can you do.

The main reason for removing this feature by default is to support
use-cases where Finit runs as the init for container apps that can read
/proc -- we do not want them to use the init args from the host.

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

doc/cmdline.md

@@ -6,6 +6,8 @@ passed on the Linux kernel command line.  Not limited to Finit.
 
 The `bool` setting is one of `on, off, true false, 1, 0`.
 
+> **NOTE:** remember to use `--` to separate kernel parameters from
+> parameters to init.  E.g., `init=/sbin/finit -- finit.debug rescue`
 
 * `debug`: Enable kernel debug.  Debug messages are printed to the
     console until Finit starts up.