#61 Patching using etoolbox or xpatch

[Markus Kohm]

In scrkernel-footnotes.dtx package etoolbox is used to patch \@makefnmark, which is in most cases enough (but maybe in future not). In scrkernel-paragraphs.dtx package xpatch is used to patch \deferred@thm@head. Maybe using xpatch both times would be better, to make the first patch robuster.

[Markus Kohm]

The code in scrkernel-footnotes.dtx is used only in scrextend. I've changed it in [r4245] to use xpatch if available, try also etoolbox of only this one is available and use hard coded redefinition in other cases.