server: Allow -D nbdkit.* debug flags for the core server.

These work like plugin/filter debug flags, but apply to the internals
of the server.

docs/nbdkit.pod

@@ -177,6 +177,13 @@ Display brief command line usage information and exit.
 Set the plugin or filter Debug Flag called C<FLAG> to the integer
 value C<N>.  See L<nbdkit-plugin(3)/Debug Flags>.
 
+=item B<-D> nbdkit.FLAG=N
+
+=item B<--debug> nbdkit.FLAG=N
+
+Set the nbdkit server Debug Flag called C<FLAG> to the integer value
+C<N>.
+
 =item B<--dump-config>
 
 Dump out the compile-time configuration values and exit.

docs/synopsis.txt

@@ -1,4 +1,4 @@
-nbdkit [-D|--debug PLUGIN|FILTER.FLAG=N]
+nbdkit [-D|--debug PLUGIN|FILTER|nbdkit.FLAG=N]
        [-e|--exportname EXPORTNAME] [--exit-with-parent]
        [--filter FILTER ...] [-f|--foreground]
        [-g|--group GROUP] [-i|--ipaddr IPADDR]

server/main.c

@@ -613,6 +613,9 @@ main (int argc, char *argv[])
     free (t);
   }
 
+  /* Apply nbdkit.* flags for the server. */
+  apply_debug_flags (NULL, "nbdkit");
+
   /* Check all debug flags were used, and free them. */
   free_debug_flags ();
 

server/nbdkit.syms

@@ -67,6 +67,8 @@
     nbdkit_vdebug;
     nbdkit_verror;
 
+    nbdkit_debug_*;
+
   # Everything else is hidden.
   local: *;
 };