Introduce a new 'ptrlen' type.

This wraps up a (pointer, length) pair into a convenient struct that
lets me return it by value from a function, and also pass it through
to other functions in one go.

Ideally quite a lot of this code base could be switched over to using
ptrlen in place of separate pointer and length variables or function
parameters. (In fact, in my personal ideal conception of C, the usual
string type would be of this form, and all the string.h functions
would operate on ptrlens instead of zero-terminated 'char *'.)

For the moment, I'm just introducing it to make some upcoming
refactoring less inconvenient. Bulk migration of existing code to
ptrlen is a project for another time.

Along with the type itself, I've provided a convenient system of
including the contents of a ptrlen in a printf; a constructor function
that wraps up a pointer and length so you can make a ptrlen on the fly
in mid-expression; a function to compare a ptrlen against an ordinary
C string (which I mostly expect to use with string literals); and a
function 'mkstr' to make a dynamically allocated C string out of one.
That last function replaces a function of the same name in sftp.c,
which I'm promoting to a whole-codebase facility and adjusting its
API.

pageant.c

@@ -556,7 +556,7 @@ void pageant_handle_msg(BinarySink *bs,
 	    key = snew(struct ssh2_userkey);
             key->data = NULL;
             key->comment = NULL;
-            key->alg = find_pubkey_alg_len(alglen, alg);
+            key->alg = find_pubkey_alg_len(make_ptrlen(alg, alglen));
 	    if (!key->alg) {
                 pageant_failure_msg(bs, "algorithm unknown", logctx, logfn);
 		goto add2_cleanup;