libexport.a: Prepare init_subnetwork() for IPv6 support

libexport.a: Prepare init_subnetwork() for IPv6 support

Retire the slash32 logic in inet_netmask() in favor of a more generic
netmask parser that can support IPv6 addresses.

If an invalid IP address string is given to inet_addr(3), it returns
INADDR_NONE, which is actually a "valid" address (255.255.255.255).
We're none the wiser to the substitution until something breaks later.

This patch provides better sanity checking of the parsed address, now
that such an error can be reported to client_init()'s callers.
We can also check the prefixlen value a little more carefully as well.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>