CHARSET PROPERLY

Tables like performance_schema.session_variables can print string values.

Before this fix, string values expressed in a character set different
than UTf8 would be truncated or incorrect.

This affects for example system variables expressed using
the file system character set, like character_sets_dir.

With the following options:
--character-set-filesystem=latin1
--character-sets-dir=<value in latin1 here>
the value for character-sets-dir is printed incorrectly,
because the latin1 value is printed as UTF8.

The root cause is that string values for system variables are
represented as a binary buffer internally, but the associated
CHARSET_INFO is lost, later assumed to be UTF8.

The fix is to preserve (CHARSET_INFO, string value, string value length)
together when representing a system variable in the performance schema,
and use the proper character set when populating the performance_schema
table.