code_env: use feature test for PR_GET_NAME support

This eliminates the FreeBSD-specific test for `prctl` and instead relies
on a feature test which is significantly easier to manage.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
doc: add dev doc to collect porting notes

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>