dup, dup2, fcntl: support a directory fd on OS/2 kLIBC
On OS/2 kLIBC, dup(), dup2() and fcntl() do not work on a directory
fd.
* lib/dup.c (dup_nothrow): New.
* lib/dup2.c (klibc_dup2dirfd): New. dup2() for a directory fd.
(klibc_dup2): New.
* lib/fcntl.c (klibc_fcntl): New.
* m4/dup.m4 (gl_FUNC_DUP): Check if dup() works on a directory fd.
* m4/dup2.m4 (gl_FUNC_DUP2): Check if dup2() works on a directory fd.
* m4/fcntl.m4 (gl_FUNC_FCNTL): Check if F_DUPFD works on a directory
fd.