rc: Honor ${name}_env when a custom *_cmd is defined (e.g., start_cmd)

A user may set ${name}_env variable in rc.conf(5) in order to set additional
environment variables for a service command. Unfortunately, at the moment
this variable is only honored when the command is specified via the command
variable. Those additional environment variables coming from ${name}_env
are never set if the service is started via the ${rc_arg}_cmd variable (for
example start_cmd).

PR: 239692
Reviewed by: bcr, jilles
Approved by: src (jilles)
Differential Revision: https://reviews.freebsd.org/D21228