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/file_access_unix.cpp

@@ -30,24 +30,20 @@
 
 #include "file_access_unix.h"
 
-#if defined(UNIX_ENABLED) || defined(LIBC_FILEIO_ENABLED)
+#if defined(UNIX_ENABLED)
 
 #include "core/os/os.h"
 #include "core/string/print_string.h"
 
+#include <errno.h>
+#include <fcntl.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 
-#include <errno.h>
-
 #if defined(UNIX_ENABLED)
 #include <unistd.h>
 #endif
 
-#ifndef ANDROID_ENABLED
-#include <sys/statvfs.h>
-#endif
-
 #ifdef MSVC
 #define S_ISREG(m) ((m)&_S_IFREG)
 #include <io.h>
@@ -56,12 +52,6 @@
 #define S_ISREG(m) ((m)&S_IFREG)
 #endif
 
-#ifndef NO_FCNTL
-#include <fcntl.h>
-#else
-#include <sys/ioctl.h>
-#endif
-
 void FileAccessUnix::check_errors() const {
 	ERR_FAIL_COND_MSG(!f, "File must be opened before use.");
 
@@ -131,13 +121,8 @@ Error FileAccessUnix::open_internal(const String &p_path, int p_mode_flags) {
 	int fd = fileno(f);
 
 	if (fd != -1) {
-#if defined(NO_FCNTL)
-		unsigned long par = 0;
-		ioctl(fd, FIOCLEX, &par);
-#else
 		int opts = fcntl(fd, F_GETFD);
 		fcntl(fd, F_SETFD, opts | FD_CLOEXEC);
-#endif
 	}
 
 	last_error = OK;
@@ -339,4 +324,4 @@ FileAccessUnix::~FileAccessUnix() {
 	_close();
 }
 
-#endif // UNIX_ENABLED || LIBC_FILEIO_ENABLED
+#endif // UNIX_ENABLED