sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Fix linker error in libuutil with recent LLVM

Not marking the function as static can result in a linker error:
undefined reference to __assfail [--no-allow-shlib-undefined]
I noticed this error after updating our CHERI LLVM to the latest upstream
LLVM HEAD revision.

This change effectively reverts r329984 and marks dmu_buf_init_user as
static (which keeps the GCC build happy).

Reviewed By: #zfs, asomers, freqlabs, mav
Differential Revision: https://reviews.freebsd.org/D25663

Don't declare __assfail as static

It gets called by dmu_buf_init_user, which is inline but not static. So it
needs global linkage itself.

Reported by: GCC-6
MFC after: 17 days
X-MFC-With: 329722

make userland __assfail from opensolaris compat honor 'aok' variable

This should allow zdb -A option to actually make difference.

MFC after: 2 weeks

Add assfail() and assfail3() to the opensolaris module.
Remove obsoleted intermediate cddl/compat/opensolaris/sys/debug.h.

MFC after: 2 weeks