It now returns an int

Thanks to Andrea Chiumenti for the bug report

Deprecate ISYS:*ENVIRON* too, better use ISYS:OS-ENVIRON

ISYS:WTERMSIG* returns the signal as keyword

Move SIZEOF to early.lisp, since the compile-time side-effects of DEFFOLDABLE are not guaranteed when used in the same file

We need to manually specify the arglist of the aliased function for
the moment

Make it an alias of CFFI:FOREIGN-TYPE-SIZE and declare it foldable

The second argument to lfp_open is of type lfp-open-flags, not int