Mark many more function (and some objects) static

I noticed commit db3b531e2cab765a00475054d2e9046c9d0437d3 in the history
where Simon added a bunch of "static" qualifiers.  That suggested that
consistently marking internal functions "static" is desirable, so I
tried a build using GCC's -Wmissing-declarations, which requires prior
declaration (presumed to be in a header file) of all global functions.

This commit makes the GTK build clean under GCC's
-Wmissing-declarations.  I've also adding "static" to a few obviously
internal objects, but GCC doesn't complain about those so I certainly
haven't got them all.

sort.c

@@ -93,14 +93,14 @@ void arraysort_fn(void *array, size_t nmemb, size_t size,
 #include <stdlib.h>
 #include <time.h>
 
-int testcmp(const void *av, const void *bv, void *ctx)
+static int testcmp(const void *av, const void *bv, void *ctx)
 {
     int a = *(const int *)av, b = *(const int *)bv;
     const int *keys = (const int *)ctx;
     return keys[a] < keys[b] ? -1 : keys[a] > keys[b] ? +1 : 0;
 }
 
-int resetcmp(const void *av, const void *bv)
+static int resetcmp(const void *av, const void *bv)
 {
     int a = *(const int *)av, b = *(const int *)bv;
     return a < b ? -1 : a > b ? +1 : 0;