Log Message:
mlterm: force consistent builds with posix_openpt(3) on NetBSD

Work around the fact this package tests whether posix_openpt(3) works at
runtime. This fails in typical sandboxed build environments (including
TNF's used for official binary builds), which causes a fallback to use
"old-style" BSD ptys, which can result in runtime failures.

Addresses PR pkg/56936. Needs monitoring to ensure official TNF binary
packages now behave as expected.