vt: fix double-click word selection for first/last word on line

Previously when double-clicking on the first word on a line we would
select from the cursor position to the end of the word, not from the
beginning of the line. Similarly, when double-clicking on the last word
on a line we would select from the beginning of the word to the cursor
position rather than the end of the line.

This is because we searched backward or forward for a space character to
mark the beginning or end of a word. Now, use the beginning or end of
the line if we do not find a space.

PR: 261553
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 692bb3f0291b21337eb9a778f71a5b97a47e4c11)
(cherry picked from commit 521dbfd6b1085511769c419d44f11842e92067f5)
(cherry picked from commit a861b27a45b0fc76bcb0dcf8ca53f1a831a13c2f)