Implement `INCLUDE_ONCE` directive

[sukus21]

Implements INCLUDE_ONCE, as suggested in #1478.

This behaves like regular include the first time a file is included, except the name of the file is added to a list. The next time using INCLUDE_ONCE on that file, the file will not be included. INCLUDE_ONCE does not take symlinks into account. The string that is registered, is the one entered into the directive.

This is effectively an include guard, not unlike C/C++ #PRAGMA ONCE.