Fix nscd readlink argument aliasing (bug 22446).

Current GCC mainline detects that nscd calls readlink with the same
buffer for both input and output, which is not valid (those arguments
are both restrict-qualified in POSIX).  This patch makes it use a
separate buffer for readlink's input (with a size that is sufficient
to avoid truncation, so there should be no problems with warnings
about possible truncation, though not strictly minimal, but much
smaller than the buffer for output) to avoid this problem.

Tested compilation for aarch64-linux-gnu with build-many-glibcs.py.

	[BZ #22446]
	* nscd/connections.c (handle_request) [SO_PEERCRED]: Use separate
	buffers for readlink input and output.

(cherry picked from commit 49b036bc)