loader: Set twiddle globaldiv to 16 by default

Booting FreeBSD on an EC2 c5.xlarge instance, the loader "twiddles"
810 times over the course of 510 ms, a rate of 1.59 kHz. Even accepting
that many systems are slower than this particular VM and will take
longer to boot (especially if using spinning-rust disks), this seems
like an unhelpfully large amount of twiddling when compared to the
~60 Hz frame rate of many displays; printing the twiddles also consumes
roughly 10% of the boot time on the aforementioned VM.

Setting the default globaldiv to 16 dramatically reduces the time spent
printing twiddles to the console while still twiddling at roughly 100
Hz; this should be ample even for systems which take longer to boot and
consequently twiddle slower.

Note that this can adjusted via the twiddle_divisor variable in
loader.conf, but that file is not processed until nearly halfway
through the loader's runtime.

Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D32163

(cherry picked from commit 7457840230c5a470ee5df8abed6ab59c4d008a45)