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/unix/dir_access_unix.cpp

@@ -30,7 +30,7 @@
 
 #include "dir_access_unix.h"
 
-#if defined(UNIX_ENABLED) || defined(LIBC_FILEIO_ENABLED)
+#if defined(UNIX_ENABLED)
 
 #include "core/os/memory.h"
 #include "core/os/os.h"
@@ -41,10 +41,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-
-#ifndef ANDROID_ENABLED
 #include <sys/statvfs.h>
-#endif
 
 #ifdef HAVE_MNTENT
 #include <mntent.h>
@@ -475,17 +472,12 @@ Error DirAccessUnix::create_link(String p_source, String p_target) {
 }
 
 uint64_t DirAccessUnix::get_space_left() {
-#ifndef NO_STATVFS
 	struct statvfs vfs;
 	if (statvfs(current_dir.utf8().get_data(), &vfs) != 0) {
 		return 0;
 	}
 
 	return (uint64_t)vfs.f_bavail * (uint64_t)vfs.f_frsize;
-#else
-	// FIXME: Implement this.
-	return 0;
-#endif
 }
 
 String DirAccessUnix::get_filesystem_type() const {
@@ -516,4 +508,4 @@ DirAccessUnix::~DirAccessUnix() {
 	list_dir_end();
 }
 
-#endif // UNIX_ENABLED || LIBC_FILEIO_ENABLED
+#endif // UNIX_ENABLED