hurd: Fix linknamespace of spawni

	* include/unistd.h (__confstr): Add prototype and hidden prototype.
	* posix/confstr.c (confstr): Rename to __confstr.
	(__confstr): Add hidden def.
	(confstr): Add weak alias for __confstr.
	* sysdeps/mach/hurd/spawni.c (__spawni): Call __confstr instead of
	confstr.