posix: netdb: provide NI_MAXSERV for reasonable app defaults

The `<netdb.h>` header typically defines `NI_MAXSERV` as a
reasonable default buffer size to use in applications that
use `getnameinfo()` to query a service name.

Most GNU and BSD systems define it by default so provide
it as a convenience so applications and 3rd-party libraries
do not encounter a compile error.

Signed-off-by: Chris Friedt <cfriedt@meta.com>