Remove dirfd definition for spawnProcessPosix

[dd86k]

NetBSD sources define dirfd as a macro, and not a function, in (NetBSD-src) include/dirent.h, as:

#define	dirfd(dirp)	((dirp)->dd_fd)

This commit defines a macro (well, static function) akin to NetBSD's macro. Without it, there is a linker error for an undefined symbol for dirfd.

The cast is done as-is because the current ABI guarantees dd_fd as the first field of _dirdesc (with typedef struct _dirdesc DIR;), and DIR structure in Druntime is only defined as an alias to void*.

It's required to build DUB, because it spawns processes. I have compiled and used DUB on my end. It is in a working state and was able to compile my ddhx project.

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10976"

[dd86k]

This PR is now waiting on dlang/dmd#22750 (dirfd definition in core.sys.posix.dirent).

[thewilsonator]

dependant PR merged