jail: reworks & cleanups

jail: reworks & cleanups

-use EXIT_SUCCESS/EXIT_FAILURE (not -1)
-parse every option in main, put them in opts struct
-add CLONE_NEWIPC to the clone() call (it's already compiled in openwrt kernel)
-return the exit status of the jailed process, or the num of the signal that killed it
-add missing options to usage()
-add a warning in usage() about ujail security
-debug option can now take an int as parameter (~debug level),
  with -d2 you now activate "LD_DEBUG=all" for exemple
-do not depend on libpreload-seccomp.so if -S is not present
-there is now only one ujail process instead of two

jail creation is now as follow:
1) create jail root dir (mkdir)
2) create new namespace (clone)
(in the parent wait for the child with uloop)
3) build the jail root fs (mount bind all the libs/bins ...),
pivot_root and mount special fs (procfs, sysfs) (build_jail_fs())
4) build envp (LD_PRELOAD the seccomp helper or ...)
5) drop capabilities (next patch)
6) execve the jailed bin
7) remove jail root dir (once child is dead)

there is no need to umount anything because we are already in a namespace

Todo:
-allow signals from the parent to the child

Feature request:
-when we add a file or dir, detect if it's an exec and add it's dependencies

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>