MFC r313547, r313777: fix mouse selection when vt(4) scrolls

r313547 (ray, submitted by hselasky):

o Reset mouse selection when new lines reach selection lines.
o Fix how selection handled on display.

r313777 (rpokala):

Un-break vt(4) for {powerpc,powerpc64,sparc64} LINT kernel builds

The {powerpc,powerpc64,sparc64} LINT kernel builds fail with this error:

sys/dev/vt/vt_buf.c:198: warning: 'vtbuf_htw' defined but not used

Move vtbuf_htw() inside the '#if SC_NO_CUTPASTE' block where it belongs, and
put it in the proper order.

This fixes the immedate issue w/ vt(4), but all three then fail on different
issues.

PR: 211922
Relnotes: Yes