Add lots of missing 'static' keywords.

A trawl through the code with -Wmissing-prototypes and
-Wmissing-variable-declarations turned up a lot of things that should
have been internal to a particular source file, but were accidentally
global. Keep the namespace clean by making them all static.

(Also, while I'm here, a couple of them were missing a 'const': the
ONE and ZERO arrays in sshcrcda.c, and EMPTY_WINDOW_TITLE in
terminal.c.)

mpint.c

@@ -878,7 +878,7 @@ unsigned mp_eq_integer(mp_int *x, uintmax_t n)
     return 1 ^ normalise_to_1(diff);   /* return 1 if diff _is_ zero */
 }
 
-void mp_neg_into(mp_int *r, mp_int *a)
+static void mp_neg_into(mp_int *r, mp_int *a)
 {
     mp_int zero;
     zero.nw = 0;