core: add varlink Reload/Reexecute methods (#39561)

Same as the D-Bus ones

diff --cc src/core/dbus-manager.c
Simple merge

diff --cc src/core/manager.c
Simple merge

diff --cc src/core/manager.h
Simple merge

diff --cc src/core/varlink-manager.c
Simple merge

diff --cc src/shared/varlink-io.systemd.Manager.c
Simple merge