capsicum-test: Remove user-space tests

The separate capsicum-test repository now holds the user-space
tests (for ease of running them on other Capsicum-capable OSes),
so remove the user-space tests in the kernel tree.

The kt.c driver for the kernel-space internal unit tests is
unaffected by this commit.
capsicum: code tidy up

None of the changes in this commit should change behaviour, they just
restructure the code for ease of future maintenance.

Object capability functionality:
 - export capability constants to userspace
 - remove security/capsicum_int.h and fold unit test
  (security/capsicum_test.c) in with the code for now
 - restructure main code file (security/capsicum.c)
 - make function naming consistent
 - make functions static that don't need to be externally visible

Process descriptor functionality (fs/procdesc.c and friends):
 - add exported procdesc.h holding PD_DAEMON
 - make local variable & function naming consistent
 - remove fs/procdesc_int.h
 - remove fs/procdesc_test.c (as the userspace test cover all its
   functionality)
capsicum: add rights checks for process descriptor syscalls

 - pdkill() requires CAP_PDKILL
 - pdgetpid() requires CAP_PDGETPID
 - pdwait$() requires CAP_PDWAIT
capsicum: implement pdwait4()

After checks, convert the process descriptor to a pid_t and
forward the call onto sys_wait4 as an initial implementation.