lib: postpone the sysrepo plugin initialization

lib: postpone the sysrepo plugin initialization

From Sysrepo's documentation:
"Note: do not use fork() after creating a connection. Sysrepo
internally stores PID of every created connection and this way a
mismatch of PID and connection is created".

Introduce a new "frr_very_late_init" hook in libfrr that is only
called after the daemon is forked (when the '-d' option is used)
and after the configuration is read. This way we can initialize
the sysrepo plugin correctly even when the daemon is daemonized,
and after the Sysrepo CLI commands are processed (only "debug
northbound client sysrepo" for now).

Fixes #7062

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>