allow to set certain client-side capabilities

[skoef]

Besides the set of default capabilities and the one capability based on server support, with this PR it is also possible to set certain client flags as well during handshake. Only certain flags make sense, below is described why a wasn't added to the list:

• CLIENT_CONNECT_WITH_DB - already being handled elsewhere in the code
• CLIENT_NO_SCHEMA- flag has no client-side use
• CLIENT_COMPRESS - this would imply the library should be able to handle compression, which it currently doesn't
• CLIENT_ODBC - this flag was deprecated server side since MySQL 3.22
• CLIENT_LOCAL_FILES - this would imply the libary would support loading local files into the server, which it currently doesn't
• CLIENT_INTERACTIVE - has no function in the client-side code
• CLIENT_SSL - already being handled elsewhere in the code
• CLIENT_IGNORE_SIGPIPE - this is only relevant when using libmysqlclient
• CLIENT_CONNECT_ATTRS - this would imply the library sends attributes during handshake, which it currently doesn't
• CLIENT_CAN_HANDLE_EXPIRED_PASSWORDS - not supported by the library at all
• CLIENT_SESSION_TRACK - not supported by the library at all
• CLIENT_DEPRECATE_EOF - not supported by the library at all

Client capabilities should be set during connecting as option function.

[skoef]

The new functions SetCapability and UnsetCapability are similar to the functions introduced in #593 for setting these capabilities on connections server-side.