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/os_unix.cpp

@@ -137,10 +137,8 @@ void OS_Unix::initialize_core() {
 	DirAccess::make_default<DirAccessUnix>(DirAccess::ACCESS_USERDATA);
 	DirAccess::make_default<DirAccessUnix>(DirAccess::ACCESS_FILESYSTEM);
 
-#ifndef NO_NETWORK
 	NetSocketPosix::make_default();
 	IPUnix::make_default();
-#endif
 
 	_setup_clock();
 }
@@ -170,6 +168,7 @@ Error OS_Unix::get_entropy(uint8_t *r_buffer, int p_bytes) {
 		left -= chunk;
 		ofs += chunk;
 	} while (left > 0);
+// Define this yourself if you don't want to fall back to /dev/urandom.
 #elif !defined(NO_URANDOM)
 	int r = open("/dev/urandom", O_RDONLY);
 	ERR_FAIL_COND_V(r < 0, FAILED);