dlclose() plugins on shutdown

We suppressed/ignored the warning about leaking the dlopen handles long
ago in Coverity, since the plugins stay loaded for the life of the
daemon.  However, with the current fork->execve model used for
"replace", I can't be sure that leaking unclosed dlopen handles doesn't
cause some kind of actual resource leak, especially on platforms I don't
regularly test.  Modern Linux/glibc doesn't show any visible leakage
from it, but better safe than sorry since it's a simple fix.

include/gdnsd/plugapi.h

@@ -124,6 +124,7 @@ typedef void (*gdnsd_start_monitors_cb_t)(struct ev_loop* mon_loop);
 //  pointers for all of the possibly-documented callbacks
 typedef struct {
     const char* name;
+    void* handle;
     bool config_loaded;
     gdnsd_load_config_cb_t load_config;
     gdnsd_map_res_cb_t map_res;

libgdnsd/plugapi.c

@@ -249,6 +249,8 @@ static plugin_t* gdnsd_plugin_load(const char* pname)
                       pname, apiv_bopt, this_bopt);
     }
 
+    plug->handle = pptr;
+
 #   define PSETFUNC(x) plug->x = (gdnsd_ ## x ## _cb_t)plugin_dlsym(pptr, pname, #x);
     PSETFUNC(load_config)
     PSETFUNC(map_res)
@@ -264,8 +266,6 @@ static plugin_t* gdnsd_plugin_load(const char* pname)
     PSETFUNC(start_monitors)
 #   undef PSETFUNC
 
-    // leak of dlopen() handle "pptr" here is intentional.  The code has no further
-    //   use for it at this point, and we never dlclose() the plugins...
     return plug;
 }
 
@@ -327,4 +327,7 @@ void gdnsd_plugins_action_exit(void)
     for (unsigned i = 0; i < num_plugins; i++)
         if (plugins[i]->exit)
             plugins[i]->exit();
+    for (unsigned i = 0; i < num_plugins; i++)
+        if (dlclose(plugins[i]->handle))
+            log_fatal("Failed to close() the '%s' plugin: %s", plugins[i]->name, dlerror());
 }