Utility function to do terminal word wrapping.

I'm planning to use this to replace some of the manually wrapped lines
in console messages.

misc.h

@@ -232,6 +232,8 @@ int string_length_for_printf(size_t);
 /* Make a ptrlen out of a constant byte array. */
 #define PTRLEN_FROM_CONST_BYTES(a) make_ptrlen(a, sizeof(a))
 
+void wordwrap(BinarySink *bs, ptrlen input, size_t maxwid);
+
 /* Wipe sensitive data out of memory that's about to be freed. Simpler
  * than memset because we don't need the fill char parameter; also
  * attempts (by fiddly use of volatile) to inhibit the compiler from

utils/CMakeLists.txt

@@ -70,6 +70,7 @@ add_sources_from_current_dir(utils
   version.c
   wcwidth.c
   wildcard.c
+  wordwrap.c
   write_c_string_literal.c
   x11authfile.c
   x11authnames.c

utils/wordwrap.c

@@ -0,0 +1,36 @@
+/*
+ * Function to wrap text to a fixed number of columns.
+ *
+ * Currently, assumes the text is in a single-byte character set,
+ * because it's only used to display host key prompt messages.
+ * Extending to Unicode and using wcwidth() could be an extension.
+ */
+
+#include "misc.h"
+
+void wordwrap(BinarySink *bs, ptrlen input, size_t maxwid)
+{
+    size_t col = 0;
+    while (true) {
+        ptrlen word = ptrlen_get_word(&input, " ");
+        if (!word.len)
+            break;
+
+        /* At the start of a line, any word is legal, even if it's
+         * overlong, because we have to display it _somehow_ and
+         * wrapping to the next line won't make it any better. */
+        if (col > 0) {
+            size_t newcol = col + 1 + word.len;
+            if (newcol <= maxwid) {
+                put_byte(bs, ' ');
+                col++;
+            } else {
+                put_byte(bs, '\n');
+                col = 0;
+            }
+        }
+
+        put_datapl(bs, word);
+        col += word.len;
+    }
+}