Improve the base64 utility functions.

The low-level functions to handle a single atom of base64 at a time
have been in 'utils' / misc.h for ages, but the higher-level family of
base64_encode functions that handle a whole data block were hidden
away in sshpubk.c, and there was no higher-level decode function at
all.

Now moved both into 'utils' modules and declared them in misc.h rather
than ssh.h. Also, improved the APIs: they all take ptrlen in place of
separate data and length arguments, their naming is more consistent
and more explicit (the previous base64_encode which didn't name its
destination is now base64_encode_fp), and the encode functions now
accept cpl == 0 as a special case meaning that the output base64 data
is wanted in the form of an unbroken single-line string with no
trailing \n.

utils/base64_encode.c

@@ -0,0 +1,40 @@
+#include "misc.h"
+
+void base64_encode_bs(BinarySink *bs, ptrlen input, int cpl)
+{
+    BinarySource src[1];
+    BinarySource_BARE_INIT_PL(src, input);
+    int linelen = 0;
+
+    while (get_avail(src)) {
+        size_t n = get_avail(src) < 3 ? get_avail(src) : 3;
+        ptrlen binatom = get_data(src, n);
+
+        char b64atom[4];
+        base64_encode_atom(binatom.ptr, binatom.len, b64atom);
+        for (size_t i = 0; i < 4; i++) {
+            if (cpl > 0 && linelen >= cpl) {
+                linelen = 0;
+                put_byte(bs, '\n');
+            }
+            put_byte(bs, b64atom[i]);
+            linelen++;
+        }
+    }
+    if (cpl > 0)
+        put_byte(bs, '\n');
+}
+
+void base64_encode_fp(FILE *fp, ptrlen input, int cpl)
+{
+    stdio_sink ss;
+    stdio_sink_init(&ss, fp);
+    base64_encode_bs(BinarySink_UPCAST(&ss), input, cpl);
+}
+
+strbuf *base64_encode_sb(ptrlen input, int cpl)
+{
+    strbuf *sb = strbuf_new_nm();
+    base64_encode_bs(BinarySink_UPCAST(sb), input, cpl);
+    return sb;
+}