rc: add ${name}_pre_start, ${name}_post_stop script support

[rozhuk-im]

Run a service-based pre_start running after setup but before start_precmd and post_stop script running after stop_postcmd.

Useful for automatic environment/chroot generation and cleanup.

[github-actions]

Thank you for taking the time to contribute to FreeBSD!

Some of files have special handling:

Important

@concussious wants to review changes to share/man/

[concussious]

Hi! So, what's wrong with precmd and postcmd?

[rozhuk-im]

It is ok, but it is hard to use it to call scripts from rc.conf:

prosody_enable="YES"
prosody_user="prosody"
prosody_group="prosody"
prosody_pidfile="/var/run/prosody.pid"
if [ 'prosody' == "${name}" ]; then
	start_precmd="/usr/local/etc/rc.d/nginx stop"
	start_postcmd="/usr/local/etc/rc.d/nginx start"
fi

It will work only if port does not use it inside rc.d script.

Same time after patch it is able to write:

turnserver_chroot="/var/run/chroot-turnserver/"
turnserver_pre_start="/root/bin/chroots/turnserver.sh start"
turnserver_post_stop="/root/bin/chroots/turnserver.sh stop"

where turnserver.sh create/destroy chroot env.

turnserver_setup is designed to generate configs and no good way use it for same kind purposes.

As administrator of services I want to able to run some code before start and after stop service, this can have many use cases to customize logic outside port, on system level.