Allow connecting to MySQL via unix sockets

[sophomeric]

If the first character of the host name is '/' then use unix_socket instead of host. Mostly to allow connections to multiple instances on a system where there may or may not be network access (mysqld_multi for example), or where there are multiple instances to allow granting permissions to only localhost.

[goatherd]

Currently I have to set up a mysql user with regular password login for munin stats to use. With socket-connection a user-auth plugin can be used without password - limited access and fewer passwords to handle.