[taadis]

macOS arm64 passes variadic arguments on the stack (Apple's AAPCS64), unlike
the other targets fun runs on. lffi.pas prepared every call with the fixed
ffi_prep_cif, so on Apple Silicon every FFI call into a variadic C function
got garbage in the tail. First victim in the stdlib: lib-ajax
(curl_easy_setopt is variadic) -- HTTP never worked on macOS, every transfer
returned rc=3 (URL_MALFORMAT). Minimal repro (before the fix):

  var p = LibC().getapi('printf', 'si:i');
  p('t=%d' & 10.toChar(), 42);   // prints  t=0

Changes:

• lffi.pas: getapi gains an optional named argument variadic: <fixed count>;
  marked bindings prepare via ffi_prep_cif_var, unmarked bindings take the
  exact old path (falling back to the fixed form if the loaded libffi
  refuses).
• lib-ajax-unx.fun: the three curl_easy_setopt bindings marked variadic: 2
  (behaviour-neutral on the SysV family; the difference between working and
  not on Darwin arm64).
• lib-ajax-lnx-test.fun un-skipped on macOS (output matches the shared
  snapshot byte for byte).

Verified: printf t=0 -> t=42 s=abc (unmarked stays t=0); lib-ajax on
Apple Silicon: file fixture, live HTTP 200, 404, saveas all correct; macOS
suite 81/81; full Linux-aarch64 suite 87/87 (SysV unaffected). The variadic
audit found only lib-proc-lnx's open -- Linux-only, left unchanged.