arosc.library: Centralized exit handling code in __arosc_startup.c
* Added autodocs for __arosc_program_start() and __arosc_program_end()
* Added functions:
* __arosc_set_errorptr()
* __arosc_set_exitjmp()
* __arosc_
* Use new functions to try to only access the aroscbase fields in __arosc_startup.c.
__arosc_nixcmain.c is still left.
* Fixed regression in vfork() that access udata after it was freed (spank me).
git-svn-id: https://svn.aros.org/svn/aros/trunk/AROS@45756 fb15a70f-31f2-0310-bbcc-cdcc74a49acc