Accept tcl 8.6 in configure's probe for tclsh.

Accept tcl 8.6 in configure's probe for tclsh.

Usually the search would find plain "tclsh" without any trouble,
but some installations might only have the version-numbered flavor
of that program.

No compatibility problems have been reported with 8.6, so we might
as well back-patch this to all active branches.

Christoph Berg