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/android/detect.py

@@ -171,7 +171,7 @@ def configure(env: "Environment"):
             "-fpic -ffunction-sections -funwind-tables -fstack-protector-strong -fvisibility=hidden -fno-strict-aliasing".split()
         )
     )
-    env.Append(CPPDEFINES=["NO_STATVFS", "GLES_ENABLED"])
+    env.Append(CPPDEFINES=["GLES_ENABLED"])
 
     if get_min_sdk_version(env["ndk_platform"]) >= 24:
         env.Append(CPPDEFINES=[("_FILE_OFFSET_BITS", 64)])
@@ -193,7 +193,7 @@ def configure(env: "Environment"):
     env.Append(LINKFLAGS="-Wl,-soname,libgodot_android.so")
 
     env.Prepend(CPPPATH=["#platform/android"])
-    env.Append(CPPDEFINES=["ANDROID_ENABLED", "UNIX_ENABLED", "NO_FCNTL"])
+    env.Append(CPPDEFINES=["ANDROID_ENABLED", "UNIX_ENABLED"])
     env.Append(LIBS=["OpenSLES", "EGL", "GLESv2", "android", "log", "z", "dl"])
 
     if env["vulkan"]: