Fix extra bracketed paste markers when pasting >8kb

Before this patch, when pasting over BUFSIZE (8192 bytes here), st would
do the following:

    \e[200~...8192 bytes...\e[201~\e[200~...remaining bytes...\e[201~

With this patch, the start marker is only sent when the offset is 0 (at
the beginning of selnotify) and the end marker is only sent when the
remaining bytes to read are 0 (at the end).

For short pastes, both conditions are true in the same iteration.

For long pastes, it removes the extra markers in the middle, keeping the
intended wrapping:

    \e[200~...8192 bytes......remaining bytes...\e[201~

Signed-off-by: Christoph Lohmann <20h@r-36.net>
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