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.

misc.h

@@ -87,6 +87,14 @@ int validate_manual_hostkey(char *key);
 
 struct tm ltime(void);
 
+ptrlen make_ptrlen(const void *ptr, size_t len);
+int ptrlen_eq_string(ptrlen pl, const char *str);
+char *mkstr(ptrlen pl);
+int string_length_for_printf(size_t);
+/* Derive two printf arguments from a ptrlen, suitable for "%.*s" */
+#define PTRLEN_PRINTF(pl) \
+    string_length_for_printf((pl).len), (const char *)(pl).ptr
+
 /* 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