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_decode.c

@@ -0,0 +1,37 @@
+#include "misc.h"
+
+void base64_decode_bs(BinarySink *bs, ptrlen input)
+{
+    BinarySource src[1];
+    BinarySource_BARE_INIT_PL(src, input);
+
+    while (get_avail(src)) {
+        char b64atom[4];
+        unsigned char binatom[3];
+
+        for (size_t i = 0; i < 4 ;) {
+            char c = get_byte(src);
+            if (get_err(src))
+                c = '=';
+            if (c == '\n' || c == '\r')
+                continue;
+            b64atom[i++] = c;
+        }
+
+        put_data(bs, binatom, base64_decode_atom(b64atom, binatom));
+    }
+}
+
+void base64_decode_fp(FILE *fp, ptrlen input)
+{
+    stdio_sink ss;
+    stdio_sink_init(&ss, fp);
+    base64_decode_bs(BinarySink_UPCAST(&ss), input);
+}
+
+strbuf *base64_decode_sb(ptrlen input)
+{
+    strbuf *sb = strbuf_new_nm();
+    base64_decode_bs(BinarySink_UPCAST(sb), input);
+    return sb;
+}