Load footmisc.sty instead of using outdated code

This is a patch I originally sent to lyx-devel in 2012 with subject
'Load footmisc.sty instead of using copied code from obsolete stblftnt.sty'.
It now takes all comments into account: It works also if the user loads the
package herself, it can be disabled by providing the footmisc feature in a
layout, and it does not use the ugly \AtBeginDocument{}.