netdb: non-blocking compat getaddrinfo_a()
netdb: non-blocking compat getaddrinfo_a()

Lauch single thread that does lookups.  It must not multi-thread
as thread-safety of libc getaddrinfo() is uncertain.

markokr/libusual,rabbitt/libusual,ifduyue/libusual,mgjv/libusual,markokr/libusual,rabbitt/libusual,markokr/libusual,ifduyue/libusual,mgjv/libusual,mgjv/libusual,markokr/libusual,ifduyue/libusual,rabbitt/libusual,rabbitt/libusual,mgjv/libusual,ifduyue/libusual