Improve support for MacOS X directory icon (update_terminal_cwd)

Improvements:
- check if update_terminal_cwd is available instead of relying on the
  kernel version
- test the feature once at init time instead of at prompt time

Note that zsh is not yet supported for that feature unless the
update_terminal_cwd has been imported in your environment.
I think it would be possible to support it, but I don't have access to
MacOS X machines, so... patches are welcome!
cpu_load: merge back Darwin and FreeBSD
Run the old PROMPT_COMMAND with eval

Because it may contain multiple commands separated with ';'.
See #196.
Remove update_terminal_cwd from _LP_OLD_PROMPT_COMMAND

... as we now have special handling for that feature of the terminal in
_LP_TERM_IPDATE_DIR