MFC r327167:

Remove obsolete register keyword from opensolaris's sysmacros.h. When
compiling zfsd with recent clang, it leads to a warning about the
register storage class being incompatible with C++17.

MFC r277300 (by smh): Mechanically convert cddl sun #ifdef's to illumos

Since the upstream for cddl code is now illumos not sun, mechanically
convert all sun #ifdef's to illumos #ifdef's which have been used in all
newer code for some time.

Also do a manual pass to correct the use if #ifdef comments as per style(9)
as well as few uses of #if defined(__FreeBSD__) vs #ifndef illumos.

MFC r284591 (by avg): illums compat: use flsl/flsll for highbit/highbit64

Do that only when when fast inline versions are available.
At the moment that can be the case only in the kernel and not for all
platforms.

The original code uses the binary search and that's kept as a fallback.
This is a micro optimization.

MFC r264669: MFV r264666:

4374 dn_free_ranges should use range_tree_t

illumos/illumos-gate@bf16b11e8deb633dd6c4296d46e92399d1582df4

MFC r284591 (by avg): illums compat: use flsl/flsll for highbit/highbit64

Do that only when when fast inline versions are available.
At the moment that can be the case only in the kernel and not for all
platforms.

The original code uses the binary search and that's kept as a fallback.
This is a micro optimization.

MFC r264669: MFV r264666:

4374 dn_free_ranges should use range_tree_t

illumos/illumos-gate@bf16b11e8deb633dd6c4296d46e92399d1582df4