Rename xmsprintf() to just xmprintf().

Partly because there's no supplied target string ala sprintf, and partly
because I can never remember what order the m and s go in.
diff --git a/lib/xwrap.c b/lib/xwrap.c
index ae296cd..2ecffaa 100644
--- a/lib/xwrap.c
+++ b/lib/xwrap.c
@@ -66,7 +66,7 @@
 }
 
 // Die unless we can allocate enough space to sprintf() into.
-char *xmsprintf(char *format, ...)
+char *xmprintf(char *format, ...)
 {
   va_list va, va2;
   int len;