Add UMASK as a configuration parameter in [mysqld]

UMASK for the mysql user is mostly used for things like allowing third-party binlog readers like GoldenGate access to normally protected datadir files.

It seems like we are working hard to preserve this definition in the service, and having concerns that installation tools should not have to do the work of reading service configuration directives in different service management systems.

If instead of putting UMASK in the service definition or the user's environment, we treated it as a server configuration directive and put it in a parameter in the [mysqld] section of the server's own configuration files, these concerns would go away, and instead of a complicated exception (for DBAs) requiring knowledge of administrative operating system details to configure this option, the DBA would only need to apply a configuration parameter like any other.