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/build.md

@@ -53,6 +53,10 @@ Below are a few of the main switches to configure:
   built-ins (.o files) and all external libraries, except the C library
   will be linked statically.
 
+* `--enable-kernel-cmdline`: Enable Finit pre-4.1 parsing of init args from
+  `/proc/cmdline`, this is not recommended since Finit may be running as the
+  init for container apps that can see the host's `/proc` filesystem
+
 * `--enable-alsa-utils-plugin`: Enable the optional `alsa-utils.so` sound plugin.
 
 * `--enable-dbus-plugin`: Enable the optional D-Bus `dbus.so` plugin.

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.