Use SIGWINCH to update terminal size

Instead of reading the terminal size on every status update, register
for SIGWINCH to read and store the size when it changes.

This reapplies I555ad21a31a2c924ab0ca681e0c8f00df42a370a with a fix
for a race condition in TestSmartStatusOutputWidthChange.

Test: status_test.go
Change-Id: If342cb4cc8e4ed57af9e3bb417758348c9c41247