Assorted further migration to ptrlen.

The local put_mp_*_from_string functions in import.c now take ptrlen
(which simplifies essentially all their call sites); so does the local
function logwrite() in logging.c, and so does ssh2_fingerprint_blob.

cmdgen.c

@@ -1002,7 +1002,7 @@ int main(int argc, char **argv)
 		} else {
 		    assert(ssh2blob);
 		    fingerprint = ssh2_fingerprint_blob(
-                        ssh2blob->s, ssh2blob->len);
+                        ptrlen_from_strbuf(ssh2blob));
 		}
 	    }