server: Allow -D debug flags to contain dots for namespacing.

This is just a convenience.  Either of:

 -D myplugin.foo_bar=1
 -D myplugin.foo.bar=1

correspond to the same plugin variable "myplugin_debug_foo_bar".

docs/nbdkit-plugin.pod

@@ -1329,6 +1329,14 @@ You should only use this feature for debug settings.  For general
 settings use ordinary plugin parameters.  Debug Flags can only be C
 ints.  They are not supported by non-C language plugins.
 
+For convenience C<'.'> characters are replaced with C<'_'> characters
+in the variable name, so both of these parameters:
+
+ -D myplugin.foo_bar=1
+ -D myplugin.foo.bar=1
+
+correspond to the plugin variable C<myplugin_debug_foo_bar>.
+
 =head1 INSTALLING THE PLUGIN
 
 The plugin is a C<*.so> file and possibly a manual page.  You can of

server/debug-flags.c

@@ -47,12 +47,20 @@ static char *
 name_of_debug_flag (const char *name, const char *flag)
 {
   char *var;
+  size_t i;
+  int len;
 
-  if (asprintf (&var, "%s_debug_%s", name, flag) == -1) {
+  if ((len = asprintf (&var, "%s_debug_%s", name, flag)) == -1) {
     perror ("asprintf");
     exit (EXIT_FAILURE);
   }
 
+  /* If there are any '.'s remaining in the name, convert them to '_'. */
+  for (i = 0; i < (size_t) len; ++i) {
+    if (var[i] == '.')
+      var[i] = '_';
+  }
+
   return var;                   /* caller frees */
 }