sigaction(2): Extend description of async-signal safe functions:

* Improve description when unsafe functions are unsafe.
* Add various safe functions from POSIX.1-2008 and Austin Group issue #692.
sigaction(2): *at system calls are async-signal safe.
Add a "kern.features" MIB for 32bit support under a 64bit kernel.
dup(2): Clarify return value, in particular of dup2().