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.

man/compton.1.asciidoc

@@ -86,6 +86,9 @@ OPTIONS
 *--config* 'PATH'::
 	Look for configuration file at the path. See *CONFIGURATION FILES* section below for where compton looks for a configuration file by default.
 
+*--write-pid-path* 'PATH'::
+	Write process ID to a file.
+
 *--shadow-red* 'VALUE'::
 	Red color value of shadow (0.0 - 1.0, defaults to 0).