nss: Introduce <nss_module.h>

nss: Introduce <nss_module.h>

This provides the struct nss_module type, which combines the old
struct service_library type with the known_function tree, by
statically allocating space for all function pointers.

struct nss_module is fairly large (536 bytes), but it will be
shared across NSS databases.  The old known_function handling
had non-some per-function overhead (at least 32 bytes per looked-up
function, but more for long function anmes), so overall, this is not
too bad.  Resolving all functions at load time simplifies locking,
and the repeated lookups should be fast because the caches are hot
at this point.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>