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

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@321200 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f