HBSD NOXEC: add pax_noexec_n{x,w}(...) function, to enforce constraints

github-issue: #37
Signed-off-by: Oliver Pinter <oliver.pinter@hardenedbsd.org>
HBSD JANITOR: document some function

github-issue: #61
Signed-off-by: Oliver Pinter <oliver.pinter@hardenedbsd.org>