String alloc shouldn't convert to std::string.

As I work through making code use more C++ RAII and such, most
of the work is handling strings, especially temporaries.  As member
variables which manage string memory get turned into `std::string`,
some use cases might wind up leaking memory.  (One was found in
this change.)

By using a non-convertible-to-string result, such accidents should
be avoided.

dlib/dlib.hh

@@ -11,6 +11,13 @@
 
 #include "d_size.h"
 
+struct CharPtrNoStringConversion
+{
+	char *ptr= nullptr;
+
+	operator char *() const noexcept { return ptr; }
+};
+
 #ifdef __cplusplus
 extern "C" {
 #endif /* __cplusplus */
@@ -84,11 +91,11 @@ void dFree (void *mem);
 /*
  *- C strings -----------------------------------------------------------------
  */
-char *dStrdup(const char *s);
-char *dStrndup(const char *s, size_t sz);
-char *dStrconcat(const char *s1, ...);
+CharPtrNoStringConversion dStrdup(const char *s);
+CharPtrNoStringConversion dStrndup(const char *s, size_t sz);
+CharPtrNoStringConversion dStrconcat(const char *s1, ...);
 char *dStrstrip(char *s);
-char *dStrnfill(size_t len, char c);
+CharPtrNoStringConversion dStrnfill(size_t len, char c);
 char *dStrsep(char **orig, const char *delim);
 void dStrshred(char *s);
 char *dStriAsciiStr(const char *haystack, const char *needle);