use seq_along(x) instead of 1:length(x)

(cherry picked from commit 2fa9b5d7725bd4dba44741dc607841058e17ff85)
Do not import namespace of tcltk

We need tcltk in one function (orditkplot) and DESCRIPTION only
suggests the package. However, NAMESPACE has import(tcltk) and if
tcltk is missing, loading vegan will fail. Now we omit tcltk import
from the NAMESPACE, and orditkplot uses explicith tcltk:: prefix for
all ca. 125 tcltk commands so that vegan can be used without tcltk
package (except for orditlplot)

I had seen this occasionally in some R builds, but the change was
triggered by queries from Dave Roberts.

(cherry picked from commit 2defbae459c0ca0b241dcc48442a4341e6c45564)