Misc: --write-pid-path & small changes

- Add --write-pid-path to write process ID to a file, to help
  determining compton's process ID when -b is used.

- Add a few extra targets to query through D-Bus opts_get (version, pid,
  etc.) and through win_get, primarily for debugging.

- Add helper macros likely(), unlikely(), cmalloc(), ccalloc(),
  crealloc().

- Change w->opacity_set to opacity_t. Determine display_repr in
  session_init(), etc.