Implement name service switch modules (NSS modules). NSS modules

may be built into libc (`static NSS modules') or dynamically loaded
via dlopen (`dynamic NSS modules').  Modules are loaded/initialized
at configuration time (i.e.  when nsdispatch is called and nsswitch.conf
is read or re-read).

Make the nsdispatch(3) core thread-safe.

New status code for nsdispatch(3) `NS_RETURN', currently used to

signal ERANGE-type issues.

syslog(3) problems, don't warn/err/abort.

Try harder to avoid namespace pollution.

Implement some shims to assist in porting NSS modules written for

the GNU C Library nsswitch interface.

Sponsored by: DARPA, Network Associates Laboratories