Fix reallocarray test assumptions about errno

This test was assuming that errno was preserved through a call to
ok(), which is obviously unsafe since it uses stdio.  Preserve errno
before calling ok().
Fix typos in util/network.* (mutliple => multiple)
Include sys/types.h in xwrite.h

stddef.h isn't sufficient since the prototypes use ssize_t and
and off_t.
Fix typo in portable/reallocarray.c introductory comment
Probe for the correct spelling of sys/bitypes.h
Treat empty string as syntax error in network_addr_match

network_addr_match now always fails (returns false) if either of the
strings are the empty string.  AIX 7.1's inet_aton treats the empty
string as equivalent to 0.0.0.0, but we want to treat it as a syntax
error since it's too easy to get an empty string by accident.