sds.c: new function sdsjoin() to join strings.

src/sds.h

@@ -89,6 +89,7 @@ sds sdsfromlonglong(long long value);
 sds sdscatrepr(sds s, const char *p, size_t len);
 sds *sdssplitargs(const char *line, int *argc);
 sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen);
+sds sdsjoin(char **argv, int argc, char *sep);
 
 /* Low level functions exposed to the user API */
 sds sdsMakeRoomFor(sds s, size_t addlen);