p/mysql_isam_space_: do not munch database names

Changing database names with underscores to dashes means that database names
with underscores cannot be used.

There's no reason for this, as the plugin is not multigraph, and the munin-node
will squash the filename.
p/mysql_isam_space_: fix the special char munging

As already said by Robert Rothenberg, the github editor has some issues
being transparent on extended accented utf-8 chars.

This commit reverts the wrong changes introduced by 492a20f.
Merge branch 'fix-mysql_isam_space_-munching' into devel