Always watch/unwatch subdirectories on all systems

Simplifies and uniformize the logic on the Lua side for the
setting of directories' watches. Now we always use the methods:

systems.watch_dir_add / rm

on all the project's directories at any depth when we are not
in files limit mode.

In files limited mode the functions systems.watch_dir_add/rm are
called only on the expanded folders. The shown_subdir table is also
updated only in files limited mode.

On the C side, using the dmon library, we remove the recursive argument
from the system.watch_dir and we always call it recursively except on
Linux. At the same time the functions:

systems.watch_dir_add / rm

are provided but as dummy functions that does nothing except on Linux
where they work as before to add / remove sub-directories in the inotify
watch.

In this was on the Lua side we always act we if the watches needed to be
set for each sub-directory explicitly, independently of the system.

The important improvement introduced is that we always avoid calling
dmon_watch recursively on Linux. This latter thing is problematic with
inotify and is therefore avoided on Linux.

On the other side we simplifies the logic on the Lua side and remove
conditions based on the OS used.

src/api/system.c

@@ -584,6 +584,11 @@ static int f_get_fs_type(lua_State *L) {
   lua_pushstring(L, "unknown");
   return 1;
 }
+#else
+static int f_return_unknown(lua_State *L) {
+  lua_pushstring(L, "unknown");
+  return 1;
+}
 #endif
 
 
@@ -715,8 +720,14 @@ static int f_set_window_opacity(lua_State *L) {
 
 static int f_watch_dir(lua_State *L) {
   const char *path = luaL_checkstring(L, 1);
-  const int recursive = lua_toboolean(L, 2);
-  uint32_t dmon_flags = (recursive ? DMON_WATCHFLAGS_RECURSIVE : 0);
+  /* On linux we watch non-recursively and we add/remove each sub-directory explicitly
+   * using the function system.watch_dir_add/rm. On other systems we watch recursively
+   * and system.watch_dir_add/rm are dummy functions that always returns true. */
+#if __linux__
+  const uint32_t dmon_flags = 0;
+#else
+  const uint32_t dmon_flags = DMON_WATCHFLAGS_RECURSIVE;
+#endif
   dmon_watch_id watch_id = dmon_watch(path, dirmonitor_watch_callback, dmon_flags, NULL);
   if (watch_id.id == 0) { luaL_error(L, "directory monitoring watch failed"); }
   lua_pushnumber(L, watch_id.id);
@@ -746,6 +757,11 @@ static int f_watch_dir_rm(lua_State *L) {
   lua_pushboolean(L, dmon_watch_rm(watch_id, subdir));
   return 1;
 }
+#else
+static int f_return_true(lua_State *L) {
+  lua_pushboolean(L, 1);
+  return 1;
+}
 #endif
 
 #ifdef _WIN32
@@ -839,6 +855,10 @@ static const luaL_Reg lib[] = {
   { "watch_dir_add",       f_watch_dir_add       },
   { "watch_dir_rm",        f_watch_dir_rm        },
   { "get_fs_type",         f_get_fs_type         },
+#else
+  { "watch_dir_add",       f_return_true         },
+  { "watch_dir_rm",        f_return_true         },
+  { "get_fs_type",         f_return_unknown      },
 #endif
   { NULL, NULL }
 };