WL#12955: ADD OS USER AS CONNECTION ATTRIBUTE IN MYSQL CLIENT

Add a new connection attribute for "mysql" client which adds information about
the OS account mysql is executing as.
This lets DBAs easier to notify people about who is running time taking queries
on the server.

This is a contribution from Daniël van Eeden, see BUG#93916

FR1: the mysql command line will add 2 extra connection attributes:
FR1.1: os_user: the current operating system user name running the mysql command
line
FR1.2: os_sudouser: the contents of the environment variable SUDO_USER
FR2: os_user will be added on both unix and windows
NF2.1: os_sudouser will not be added for windows
NF3: Any other clients (libmysql based or not) will not be affected
NF4: if for some reason the os_user or os_sudouser are calculated as empty the
corresponding connection attribute will not be added.