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)
 
 /*

include/modval.h

@@ -164,12 +164,7 @@ extern Function_ptr *global;
 #define plural (*(char * (*)(int))global[PLURAL])
 #define my_ctime (*(char * (*)(time_t))global[MY_CTIME])
 #define ccspan (*(size_t (*)(const char *, int))global[CCSPAN])
-
-/* If we are in a module, undefine the previous define from ltoa to my_ltoa */
-#ifdef ltoa
-#undef ltoa
-#endif
-#define ltoa (*(char *(*)(long ))global[LTOA])
+#define my_ltoa (*(char *(*)(long ))global[LTOA])
 #define strformat (*(char *(*)(char *, const char *, int , char ))global[STRFORMAT])
 #define MatchingBracket (*(char *(*)(char *, char , char ))global[MATCHINGBRACKET])
 #define parse_number (*(int (*)(char **))global[PARSE_NUMBER])

source/scr-bx.c

@@ -97,7 +97,7 @@ int get_int_var(int var)
 	return 1;
 }
 
-char *ltoa (long foo)
+char *my_ltoa(long foo)
 {
 	static char buffer[BIG_BUFFER_SIZE/8+1];
 	char *pos = buffer + BIG_BUFFER_SIZE/8-1;