hurd: Fix `getifaddrs' and `freeifaddrs' symbol exposition

from `freeaddrinfo'.

`getifaddrs' and `freeifaddrs' are not in POSIX, they should not be
exposed along `freeaddrinfo' (through `__check_pf') which is POSIX.

	* include/ifaddrs.h (__getifaddrs, __freeifaddrs): New declarations,
	and use libc_hidden_def on them.
	* inet/ifaddrs.c (__getifaddrs, __freeifaddrs): Use libc_hidden_def on
	them.
	* sysdeps/gnu/ifaddrs.c (__getifaddrs, __freeifaddrs): Likewise.
	* inet/check_pf.c (__check_pf): Use __getifaddrs and __freeifaddrs
	instead of getifaddrs and freeifaddrs.