init_main: Use TUNABLE_INT_FETCH to initialize verbose_sysinit

It is too late to initialize verbose_sysinit via TUNABLE_INT, as
TUNABLE_INT runs at the order of SI_SUB_TUNABLES, thus any subsystems
those have order prior or equal to SI_SUB_TUNABLES are not logged.

Reviewed by: kevans
Fixes: c7962400c9a7 Add debug.verbose_sysinit tunable for VERBOSE_SYSINIT
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47907