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.

drivers/windows/dir_access_windows.cpp

@@ -309,39 +309,13 @@ Error DirAccessWindows::remove(String p_path) {
 	}
 }
 
-/*
-
-FileType DirAccessWindows::get_file_type(const String& p_file) const {
-	WCHAR real_current_dir_name[2048];
-	GetCurrentDirectoryW(2048, real_current_dir_name);
-	String prev_dir = Strong::utf16((const char16_t *)real_current_dir_name);
-
-	bool worked = SetCurrentDirectoryW((LPCWSTR)(current_dir.utf16().get_data()));
-
-	DWORD attr;
-	if (worked) {
-		WIN32_FILE_ATTRIBUTE_DATA fileInfo;
-		attr = GetFileAttributesExW((LPCWSTR)(p_file.utf16().get_data()), GetFileExInfoStandard, &fileInfo);
-	}
-
-	SetCurrentDirectoryW((LPCWSTR)(prev_dir.utf16().get_data()));
-
-	if (!worked) {
-		return FILE_TYPE_NONE;
-	}
-
-	return (attr & FILE_ATTRIBUTE_DIRECTORY) ? FILE_TYPE_
-}
-
-*/
-
 uint64_t DirAccessWindows::get_space_left() {
 	uint64_t bytes = 0;
 	if (!GetDiskFreeSpaceEx(nullptr, (PULARGE_INTEGER)&bytes, nullptr, nullptr)) {
 		return 0;
 	}
 
-	//this is either 0 or a value in bytes.
+	// This is either 0 or a value in bytes.
 	return bytes;
 }