lxc_monitord: make lxc-monitord async signal safe

lxc_monitord: make lxc-monitord async signal safe

Before lxc_monitord called lxc_monitord_cleanup() from a signal handler.  This
function calls a bunch of async signal unsafe functions and basically begs for
deadlocks. This commit switches lxc-monitord to using sigsetjmp() and
siglongjmp() in the signal handler to jump to a cleanup label that call
lxc_monitord_cleanup(). In this way, we avoid using async signal unsafe
functions.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>