sys_linux: use privops helper when running with seccomp filter

Enable the PRV_Name2IPAddress() function with seccomp support and start
the helper process before loading the seccomp filter (but after dropping
root privileges). This will move the getaddrinfo() call outside the
seccomp filter and should make it more reliable as the list of required
system calls won't depend on what glibc NSS modules are used on the
system.

configure

@@ -629,6 +629,10 @@ if [ $feat_scfilter = "1" ] && [ $try_seccomp = "1" ] && \
     'seccomp_init(SCMP_ACT_KILL);'
 then
   add_def FEAT_SCFILTER
+  # NAME2IPADDRESS shouldn't be enabled with other operations as the helper
+  # process works on one request at the time and the async resolver could
+  # block the main thread
+  priv_ops="NAME2IPADDRESS"
   EXTRA_LIBS="$EXTRA_LIBS -lseccomp"
 fi