Remove not used DBSocket default parameter and improve documentation and description

DBSocket default is /tmp/mysql.sock, but default is NULL in source and it uses libmysqlclient library's default.

(1) remove default parameter

Zabbix doesn't use /tmp/mysql.sock. It's better to remove it

(2) update documentation and description in configuration file

It's better to explain it in description. Documentation also should be updated.
https://www.zabbix.com/documentation/3.0/manual/appendix/config/zabbix_server

(3) DBport parameter

DBPort parameter also uses database client library default. MySQL Port is mentioned in the descripiotn and documentation now. It's better to update it same as DBSocket way above.