Manual DSO cleanup after renaming:

- renamed remaining os_ stuff in os/unix/ and os/bs2000/ to ap_ variants
- merged the ap_dso_xxxx non-HPUX-specific #defines
  and the HPUX-specific functions
- moved the FreeBSD-underscore hack from mod_so (where it was badly placed
  because it is a low-level stuff) to the dlsym() call in os/unix/os.c
  (where is is placed better because _there_ is the os-specific stuff)
- some minor cleanups to os.h
- additions to compat.h


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@80886 13f79535-47bb-0310-9956-ffa450edef68