fpathconf(3) and pathconf(3) can share code. There's no such
header file as <pathconf.h>. glibc/POSIX and BSD disagree about where
the _POSIX_* definitions should go.
Change-Id: I4a67f1595c9f5fbb26700a131178eedebd6bf712
The upstream intention was for this to be architecture-dependent, but it's a
lot clearer if we just have one copy.
Change-Id: I4e8310496145f9f411cd2e847c8cd023b1d758e9