sysutils/screen*: Fix off-by-one buffer overflow

FreeBSD ncurses, as of c8b9c85ee5bb, does a strncpy() of 1024 bytes into
a 1023 byte buffer supplied by screen. This section of code in ncurses
was removed in 61f66a1f4403, and is not a problem since 14.0-RELEASE.
But it is still a problem in 13-STABLE.

Thank you to dim@ for detailed analysis and initial patch to
sysutils/screen. The same patch is also applied to sysutils/screen-devel
this commit.

PR: 280868
MFH: 2024Q3