Document and simplify the ltoa() / my_ltoa() distinction

The function provided (either by #define to the module table or directly in the case of scr-bx.c)
is called my_ltoa(), and the #define in irc_std.h aliases this to ltoa().

include/irc_std.h

@@ -238,6 +238,8 @@ int ioctl (int, int, ...);
 #define strncasecmp strnicmp
 #endif
 
+/* We provide our own ltoa() rather than rely on the various non-standard ones
+ * that some platforms provide. */
 #define ltoa(a) my_ltoa(a)
 
 /*