tftp(1): Fix libedit state corruption involving signals

This bug was first reported 14 years ago. The problem was understood 8.5
years ago. A patch that is functionally identical to this one was proposed
almost 8 years ago and languished in the PR system / Bugzilla.

PR: 63197
Submitted by: lxv AT omut.org, fernando.apesteguia AT gmail.com
Reported by: freebsd AT nbritton.org