nameserv: add asynchronous resolving with POSIX threads

Run getaddrinfo()/gethostbyname() in separate thread to avoid blocking.
Only one resolving thread is running at one time, so this should work
also on systems where the functions are not thread-safe.

logging.h

@@ -77,6 +77,7 @@ typedef enum {
   LOGF_Manual,
   LOGF_Keys,
   LOGF_Logging,
+  LOGF_Nameserv,
   LOGF_Rtc,
   LOGF_Regress,
   LOGF_Sys,