etcupdate: Gracefully handle SIGINT when building trees.

[jhb]

Summary

Run the 'build_tree' function inside of a subshell and trap SIGINT to
return an error to the caller. This allows callers to gracefully
cleanup a partially created tree.

While here, redirect stdout/stderr of the subshell to the log file
instead of applying redirections individually to each command executed
while building the tree.

MFC after: 2 weeks

[imp]

oh, I think this does what my thought in another review suggested...

[dim]

usr.sbin/etcupdate/etcupdate.sh

182

Aren't you trapping SIGINT below? :)

[jhb]

usr.sbin/etcupdate/etcupdate.sh

182

Bah, I fat fingered when I updated this comment.

[jhb]

s/SIGQUIT/SIGINT/ in comment.