Update source file names in comments and docs.

Correcting a source file name in the docs just now reminded me that
I've seen a lot of outdated source file names elsewhere in the code,
due to all the reorganisation since we moved to cmake. Here's a giant
pass of trying to make them all accurate again.

unix/pageant.c

@@ -90,7 +90,7 @@ static int make_pipe_to_askpass(const char *msg)
         cloexec(pipefds[1]);
 
         /*
-         * See comment in fork_and_exec_self() in gtkmain.c.
+         * See comment in fork_and_exec_self() in main-gtk-simple.c.
          */
         execv("/proc/self/exe", (char **)args);
         execvp(progname, (char **)args);