loader: abstract boot services exiting to libefi function

Move direct call of ExitBootServices to efi_exit_boot_services. This
function sets boot_services_active to false so callers don't have to do
it everywhere (though currently only loader/bootinfo.c is affected).

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D32226

(cherry picked from commit 77e3db07898461de458caece30f42adceff222a9)