bug#33094: Error in upgrading from 5.0 to 5.1 when table contains
triggers
and
#41385: Crash when attempting to repair a #mysql50# upgraded table
with triggers.

Problem:
1. trigger code didn't assume a table name may have
a "#mysql50#" prefix, that may lead to a failing ASSERT().
2. "ALTER DATABASE ... UPGRADE DATA DIRECTORY NAME" failed
for databases with "#mysql50#" prefix if any trigger.
3. mysqlcheck --fix-table-name didn't use UTF8 as a default
character set that resulted in (parsing) errors for tables with
non-latin symbols in their names and definitions of triggers.

Fix:
1. properly handle table/database names with "#mysql50#" prefix.
2. handle --default-character-set mysqlcheck option;
if mysqlcheck is launched with --fix-table-name or --fix-db-name
set default character set to UTF8 if no --default-character-set
option given.

Note: if given --fix-table-name or --fix-db-name option,
without --default-character-set mysqlcheck option
default character set is UTF8.