init: fixed issues related to forking services

Fixed issues related to forking services into new PID + mount
namespaces.

Remounting rootfs recursively as slave when creating a service in new
PID + mount namespaces. This prevents the service from interfering with
mount points in the parent namespace.

Unmount then mount /proc instead of mounting it with MS_REMOUNT, since
MS_REMOUNT is not sufficient to update /proc to the state appropriate
for the new PID namespace. Note that the /proc mount options specified
here are not the same as those used in the default mount namespace. I
kept them consistent with those used in the code prior to this fix.

Test: Used custom sleepd service to test init 'namespace' keyword.
Tested on angler in oreo-dev - I had to add PID namespaces to the
kernel (commit ad82c662).

Change-Id: I859104525f82fef3400d5abbad465331fc3d732f