capsicum: do path_lookup check on absolute names

When walking an absolute path, the link_path_walk() function
strips off the leading "/"; however, the security_path_lookup()
check needs to be aware of the absolute path lookup.
capsicum: track forked processes that don't generate SIGCHLD

For a pdfork()ed subprocess, we need no SIGCHLD signal to be
generated on process exit so that (say) libraries can implement
functionality in sandboxed sub-processes, without the using
application being disturbed by unexpected SIGCHLD signals.

However, the set of tasks that are eligible for waitpid() is
governed by whether they have an exit_signal value or not.

So add a new per-task boolean quiet_forked indicating that a task
has been forked (not cloned) even though its exit_signal is not
present.
capsicum-test: add wrapper script for running UML under gdb

Also need to set 'handle SIGSEGV nostop noprint pass'.