terminal: use exact window size for pager (#3249)

Instead of using a fixed 100x30 window size query the operating system
for the exact size, also fixes a bug where the last line before calling
the pager is repeated twice.

pkg/terminal/out_unix.go

@@ -0,0 +1,25 @@
+//go:build linux || darwin || freebsd
+// +build linux darwin freebsd
+
+package terminal
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+type winSize struct {
+	row, col       uint16
+	xpixel, ypixel uint16
+}
+
+func (w *pagingWriter) getWindowSize() {
+	var ws winSize
+	ok, _, _ := syscall.Syscall(syscall.SYS_IOCTL, uintptr(syscall.Stdout), syscall.TIOCGWINSZ, uintptr(unsafe.Pointer(&ws)))
+	if int(ok) < 0 {
+		w.mode = pagingWriterNormal
+		return
+	}
+	w.lines = int(ws.row)
+	w.columns = int(ws.col)
+}