Improve MANPATH control

Improve MANPATH control

* Revert changes to programs/dtsearchpath/libCliSrv/UnixEnv.C
  introduced by c3cb5b8aa6997138dfa06da39142bf793baeae60
  that could have produced disappearing Application Manager
  icons on FreeBSD
* Introduce SearchPath:useSystemPath() virtual method to tell
  dtsearchpath to leave some environment variables alone.

  It is currently overriden for FreeBSD only if the MANPATH
  is empty (system default). Other operating systems
  might want to override it if they prefer to have distribution
  specific control of a search path in effect.

* Symlink /usr/dt/share/man to /usr/dt/man for FreeBSD
  This allows dtsearchpath to actually include /usr/dt/man
  in the MANPATH when MANPATH override is in effect.