add `redis-replica.sh` to create redis replication easily with `screen(1)`
add _buffer_ module
fix several bugs and add more functionalities

- fix a bug that invalidates BUFFER::lim in `buf_flip()`
- fix a bug in `buf_vprintf()` that cause buffer overflow.
- add `buf_gets()`: NOT TESTED!
- add some documentation
fix some bugs in _xerror_ module

- fix a bug that uses uninitialized `pthread_mutexattr_t` value
- fix `WRITE_NUM()` macro to deal in case of `long2str()` failure
Merge remote-tracking branch 'origin/master'
sentinel support in `redis-replica.sh`