<unistd.h>: Clean up the _POSIX2_* defines (Shells and Utilities options).

<unistd.h>: Clean up the _POSIX2_* defines (Shells and Utilities options).

* Bump the _POSIX2_C_BIND, _POSIX2_FORT_RUN and _POSIX2_UPE defines to
  200809.

* Define _POSIX2_C_DEV, _POSIX2_LOCALEDEF and _POSIX2_SW_DEV to 200809.

* Improve comments.

Things I know of that we still have to do:

* For 100% _POSIX2_UPE conformance we still need to implement at least
  sh(1)'s HISTFILE and MAILCHECK environment variables.

* For 100% _POSIX2_SW_DEV conformance we still need to implement at
  least make(1)'s -p option.