Fix Debian init script fails when socket not specified

[PeterDaveHello]

Set $socket to default /var/run/mysqld/mysqld.sock when it's not properly detected.

MySQL works fine with socket not set in the config file, however, mysqld --print-defaults won't print it, therefore mysqld_get_param() can not fetch the info properly.

cc #583

[PeterDaveHello]

Hi @delgod @EvgeniyPatlan, would you please help review/audit this PR? Thanks a lot!

[CLAassistant]

All committers have signed the CLA.

[EvgeniyPatlan]

thanks for your contribution @PeterDaveHello and sorry for delay with it.
Could you please sign license agreement so we could be able to merge these changes

[PeterDaveHello]

Sure, it's done, thanks!

[PeterDaveHello]

BTW, if you guys will like to receive more contributions, I'd be happy to help improve the shell scripts here.

[EvgeniyPatlan]

@PeterDaveHello thank you. It would be great!