On platforms where closefrom returns void (eg glibc>=2.34) the prototype for closefrom in its compat tests would cause compile errors. Remove this and have the tests pull in the compat headers in the same way as the main code. bz#3336.
[openbsd-compat/regress/strduptest.c] Add missing includes so they pass compilation with "-Wall -Werror"
Add first attempt at regress tests for compat library. ok djm@