abd_os: move headers from libzpool to libspl

[robn]

[Sponsors: Klara, Inc., Wasabi Technology, Inc.]

Motivation and Context

5b9e695 added specific userspace versions of abd_os.h and abd_impl_os.h for libzpool. However, abd.h and abd_impl.h, which include them, are packaged with libzfs, so other programs building against libzfs can fail to build, either because the headers aren't installed, or because they aren't on any standard include path.

Description

Move abd_os.h and abd_impl_os.h to libspl, where they we will be installed alongside abd.h and abd_impl.h in a known path.

Fixes #16940
Fixes #17390

How Has This Been Tested?

Installation and package building.

Before:

$ make install
...
$ find /usr/local/include -name abd\*.h
/usr/local/include/libzfs/sys/abd.h
/usr/local/include/libzfs/sys/abd_impl.h
/usr/local/include/libzpool/abd_impl_os.h
/usr/local/include/libzpool/abd_os.h

$ make -j6 deb-utils
...
$ dpkg-deb -c libzfs6-devel_2.3.99-389_amd64.deb | grep 'abd.*\.h'
-rw-r--r-- root/root      6037 2025-05-29 11:05 ./usr/include/libzfs/sys/abd.h
-rw-r--r-- root/root      3559 2025-05-29 11:05 ./usr/include/libzfs/sys/abd_impl.h
-rw-r--r-- root/root      1259 2025-05-29 11:05 ./usr/include/libzpool/abd_impl_os.h
-rw-r--r-- root/root      1278 2025-05-29 11:05 ./usr/include/libzpool/abd_os.h

$ rpm2cpio libzfs6-devel-2.3.99-389_g00360efa35.x86_64.rpm | cpio -it | grep 'abd.*\.h'
./usr/include/libzfs/sys/abd.h
./usr/include/libzfs/sys/abd_impl.h
./usr/include/libzpool/abd_impl_os.h
./usr/include/libzpool/abd_os.h

After:

$ make install
...
$ find /usr/local/include -name abd\*.h
/usr/local/include/libzfs/sys/abd.h
/usr/local/include/libzfs/sys/abd_impl.h
/usr/local/include/libspl/sys/abd_os.h
/usr/local/include/libspl/sys/abd_impl_os.h

$ make -j6 deb-utils
...
$ dpkg-deb -c libzfs6-devel_2.3.99-390_amd64.deb | grep 'abd.*\.h'
-rw-r--r-- root/root      1259 2025-05-29 10:47 ./usr/include/libspl/sys/abd_impl_os.h
-rw-r--r-- root/root      1278 2025-05-29 10:47 ./usr/include/libspl/sys/abd_os.h
-rw-r--r-- root/root      6037 2025-05-29 10:47 ./usr/include/libzfs/sys/abd.h
-rw-r--r-- root/root      3559 2025-05-29 10:47 ./usr/include/libzfs/sys/abd_impl.h

$ make -j6 rpm-utils
...
$ rpm2cpio libzfs6-devel-2.3.99-390_g1d2b185182.x86_64.rpm | cpio -it | grep 'abd.*\.h'
./usr/include/libspl/sys/abd_impl_os.h
./usr/include/libspl/sys/abd_os.h
./usr/include/libzfs/sys/abd.h
./usr/include/libzfs/sys/abd_impl.h

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.