script: Minor cleanups.

• Instead of using isatty() to decide whether to call tcgetattr(), just call tcgetattr() directly, since that's all that isatty() does anyway.
• Simplify error handling in termset(). Check for errno != ENOTTY from tcgetattr() to handle errors that may be raised while running script(1) under a debugger.

PR: 248377
Submitted by: Soumendra Ganguly <soumendraganguly@gmail.com>
MFC after: 1 week