etcupdate: Gracefully handle SIGINT when building trees.

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.

Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D29844