make a hook for user scripts during the initenv stage
make a hook for user scripts during the initenv stage

If the directory $workdir/upgrade exists with:
  post-initenv-\*
  pre-initenv-*
files, execute them before and after cbsd initenv.
It allows users to create custom pre/post-upgrade scripts, e.g for backup, notification,
migration and so on. Issue #411

cbsd/cbsd,olevole/cbsd,cbsd/cbsd,olevole/cbsd,cbsd/cbsd,olevole/cbsd,cbsd/cbsd,olevole/cbsd