misc. sql/slave.cc & co. refactor

* `get_master_version_and_clock()` de-duplicate label using fall-through
* `io_slave_killed()` & `check_io_slave_killed()`:
  * reüse the result from the level lower
  * add distinguishing docs
* `try_to_reconnect()`: extract `'` from `if`-`else`
* `handle_slave_io()`: Both `while`s have the same condition;
  looks like the outer `while` can simply be an `if`.
* `connect_to_master()`:
  * assume `mysql_errno()` is not 0 on connection error
  * utilize 0’s falsiness in the loop
  * extend docs
* `sql/sql_show.cc`: refactor SHOW ALL REPLICAS filter’s condition
* `sql/mysqld.cc`: init `master-retry-count` with `master_retry_count`

Reviewed-by: Kristian Nielsen <knielsen@knielsen-hq.org>

mysql-test/include/check-testcase.inc

@@ -163,4 +163,3 @@ if (`SELECT COUNT(*)=1 FROM INFORMATION_SCHEMA.PLUGINS WHERE PLUGIN_NAME = 'wsre
 }
 
 --enable_query_log
-