allow for large KTR_ENTRIES values by allocating ktr_buf using malloc(9)

Only during very early boot, before malloc(9) is functional (SI_SUB_KMEM),
the static ktr_buf_init is used. Size of the static buffer is determined
by a new kernel option KTR_BOOT_ENTRIES. Its default value is 1024.

This commit builds on top of r243046.

Reviewed by: alc
MFC after: 17 days