hurd: Use the new file_exec_paths RPC

From: Emilio Pozuelo Monfort <pochu27@gmail.com>
From: Svante Signell <svante.signell@gmail.com>

Pass the file paths of executable to the exec server, both relative and
absolute, which exec needs to properly execute and avertise #!-scripts.
Previously, the exec server tried to guess the name from argv[0] but argv[0]
only contains the executable name by convention.

	* hurd/hurdexec.c (_hurd_exec): Deprecate function.
	(_hurd_exec_paths): New function.
	* hurd/hurd.h (_hurd_exec): Deprecate function.
	(_hurd_exec_paths): Declare function.
	* hurd/Versions: Export _hurd_exec_paths.
	* sysdeps/mach/hurd/execve.c: Include <stdlib.h> and <stdio.h>
	(__execve): Use __getcwd to build absolute path, and use
	_hurd_exec_paths instead of _hurd_exec.
	* sysdeps/mach/hurd/spawni.c: Likewise.
	* sysdeps/mach/hurd/fexecve.c: Use _hurd_exec_paths instead of
	_hurd_exec.