amicommon: fixed sysdir.inc/do_chdir() to actually work on AROS
- fixed all Amiga-like system units to not leak the last directory
  lock while changing back to the original dir in System_exit;
  (a bug found by Marcus Sackrow, thanks!)
* renamed AOS_origDir to ASYS_origDir


git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@28537 3ad0048d-3df7-0310-abae-a5850022a9f2