kernel/fcall.c optimizations

Features:
  * removed duplicated code
  * removed `noreturn` parameter (`return_value == NULL` when the caller does not want the return value) — less arguments => more opportunities for inlining
  * reorganized macros in kernel/fcall.h a bit (for readability)
  * removed not used code

Unrelated but required changes:
  * fix spurious segfaults in `kernel/string.c` due to not initialized `use_copy`
  * updated php_phalcon.h in build/
  * added missing headers/sources to gen-build.php
  * instructed gen-build.php not to strip system headers