The documentation implied it would override "promote_command", which is
not the case.
"promote_command" is used by repmgrd to execute "repmgr standby promote"
(either directly or via a custom script).
"service_promote_command" can be set to specify a package-level service
command to promote the local PostgreSQL instance from standby to primary,
e.g. Debian's pg_ctlcluster. If set, this will be executed by "repmgr standby promote".
Also update code comments to clarify usage.
Related to GitHub #473.
It's hard to imagine a use case where this isn't desirable, but
in case, for whatever reason, the user does not wish to daemonize the
process, the command line option "--daemonize=false" can be provided.
Implements GitHub #458.
