Unix: Remove now unnecessary I/O defines, cleanup

- `LIBC_FILEIO_ENABLED` wasn't defined anywhere, even in _other platforms_.
- `NO_NETWORK` is also never defined. It probably isn't enough anyway to
  disable network APIs in the current codebase.
- `UNIX_SOCKET_UNAVAILABLE` is never defined in this code but used by some
  other platforms, clarify that.
- `NO_STATVFS` can be removed as Android supports it since API level 19,
  which is our current min SDK level. It's also only used for
  `DirAccessUnix::get_space_left()` which is anyway overridden by
  `DirAccessJAndroid::get_space_left()` so it shouldn't make a difference.
  * Fixed documentation for `DirAccess.get_space_left()`.
- `NO_FCNTL` is likely also a remnant of early Android days, in current NDK
  r23 it seems to be available. Also cleaned up unused `fcntl.h` includes.
- `NO_ALLOCA` is never defined, and we use alloca in many places now.

platform/linuxbsd/display_server_x11.cpp

@@ -49,10 +49,14 @@
 #include "drivers/gles3/rasterizer_gles3.h"
 #endif
 
+#include <dlfcn.h>
 #include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
 
 #include <X11/Xatom.h>
 #include <X11/Xutil.h>
@@ -66,17 +70,6 @@
 #define _NET_WM_STATE_REMOVE 0L // remove/unset property
 #define _NET_WM_STATE_ADD 1L // add/set property
 
-#include <dlfcn.h>
-#include <fcntl.h>
-#include <sys/stat.h>
-#include <sys/types.h>
-#include <unistd.h>
-
-//stupid linux.h
-#ifdef KEY_TAB
-#undef KEY_TAB
-#endif
-
 #undef CursorShape
 #include <X11/XKBlib.h>