String value unsharing refactored into proper function.

All the Redis functions that need to modify the string value of a key in
a destructive way (APPEND, SETBIT, SETRANGE, ...) require to make the
object unshared (if refcount > 1) and encoded in raw format (if encoding
is not already REDIS_ENCODING_RAW).

This was cut & pasted many times in multiple places of the code. This
commit puts the small logic needed into a function called
dbUnshareStringValue().

src/redis.h

@@ -1176,6 +1176,7 @@ void setKey(redisDb *db, robj *key, robj *val);
 int dbExists(redisDb *db, robj *key);
 robj *dbRandomKey(redisDb *db);
 int dbDelete(redisDb *db, robj *key);
+robj *dbUnshareStringValue(redisDb *db, robj *key, robj *o);
 long long emptyDb(void(callback)(void*));
 int selectDb(redisClient *c, int id);
 void signalModifiedKey(redisDb *db, robj *key);