restart_deps uses an on-disk queue of known file changes

apply_overlay will now record all changes into a DBM file in the
$state_dir.  in addition to the mtime checks to see if a file has
been altered, restart_deps will also check this DBM file (and clear
its contents when done).  this should improve resiliency in the
face of fatal runtime errors (why hello there, install_packages...)

also cleanup the atrociously inconsistent whitespace/bracing in
these files.