Add Pre/Post fppd start and stop hooks for plugins.

If plugins need to run pre/post scripts before or after
fppd startup or shutdown, they need to have a scripts
subdirectory.  This subdir can contain up to four scripts
which will be called at the appropriate time:

scripts/preStart.sh  - called from fppd_start before fppd is started
scripts/postStart.sh - called from fppd_start after fppd is started
scripts/preStop.sh   - called from fppd_stop before fppd is stopped
scripts/postStop.sh  - called from fppd_stop after fppd is stopped