Obsolete nfsservctl.

The Linux nfsservctl syscall was removed in Linux 3.1.  Since the
minimum kernel version for use with glibc is 3.2, the glibc wrapper
for this syscall can no longer usefully be called.  This patch makes
it into a compat symbol, not provided at all for static linking or new
ports.  (It was already the case that there was no header declaration
of this function.)

Tested for x86_64.

	* sysdeps/unix/sysv/linux/syscalls.list (nfsservctl): Make into a
	compat symbol, disabled for minimum symbol version GLIBC_2.28 and
	later.