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I've seen a lot of outdated source file names elsewhere in the code,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/x11misc.h

@@ -7,14 +7,8 @@
 
 #ifndef NOT_X_WINDOWS
 
-/*
- * x11misc.c.
- */
+/* Defined in unix/utils */
 void x11_ignore_error(Display *disp, unsigned char errcode);
-
-/*
- * gtkmisc.c
- */
 Display *get_x11_display(void);
 
 #endif