Prefer static to extern where either will do

Prefer static to extern where either will do

* src/charset.c (charset_table_size):
* src/pdumper.c (dump_private):
* src/sysdep.c (init_sigsegv):
* src/window.c (old_selected_window):
Now static.
* src/charset.c (charset_table_size):
Now int, since the value always fits in int.
* src/gtkutil.c (xg_gtk_initialized): Now present only if
HAVE_XWIDGETS, to make it clearer that this is an xwidgets
hack.  All uses changed.
* src/lread.c (ndefsubr): Remove; unused.
* src/pdumper.h: Use usual GNU indenting style for functions,
since my static-vs-extern checking hack relies on it.
(dump_public): Always declare; simpler and doesn’t hurt.
(pdumper_handle_page_fault): Remove unused decl.