The PHP_SBINDIR symbol was defined on *nix systems but never used. This
adds the constant similar to PHP_BINDIR also to PHP. On Windows it is
the value of prefix configuration when PHP was built (same value as
PHP_BINDIR).
Basically all constants are now declared via stubs. The rest of the constants are either deprecated (`SID` or `MHASH_*`) or out of interest (`__COMPILER_HALT_OFFSET__` and `PHP_CLI_PROCESS_TITLE`).