action: do not expand env vars in Exec action

...<command> argument (but still resolve tilde).

This makes it easier to write sh -c '' constructs without turning labwc
into a shell parser in order to expand environment variables, whilst
respecting single quotes and escaped characters as well as ignoring
subshells syntax like $(foo) and backticks.

Also, fix bug where buffer length+alloc get out-of-sync

docs/labwc-actions.5.scd

@@ -19,6 +19,7 @@ Actions are used in menus and keyboard/mouse bindings.
 	Execute command.  Note that in the interest of backward compatibility,
 	labwc supports <execute> as an alternative to <command> even though
 	openbox documentation states that it is deprecated.
+	Note: Tilde (~) is expanded in the command before passing to execvp()
 
 *<action name="Exit" />*
 	Exit labwc.

docs/labwc-config.5.scd

@@ -32,6 +32,8 @@ variables accordingly. It is recommended to specify keyboard layout settings and
 cursor size/theme here; see environment variable section below for details. Note
 that the environment file is treated differently by openbox where it is simply
 sourced prior to running openbox.
+Note: Tilde (~) and environment variables in the value are expanded, but
+subshell syntax and apostrophes are ignored.
 
 The *menu.xml* file defines the context/root-menus and is described in
 labwc-menu(5).