export $autoboot var when running from $boottrace_cmd

At the moment, if bootrace profiling is enabled, autoboot is not
exported to the rc scripts. This causes fsck to not check the root
filesystem. To fix this, pass _boot, rc_fast and autoboot to the
boottrace process to mimic what diretly sourcing the rc.d scripts.

PR: 278993
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1261