Add zfs_gitrev.h to the distributed sources

Commit 109d2c931020 ("Move zfs_gitrev.h to build directory") stopped
distributing zfs_gitrev.h, as it is a generated file. Add it back, with
some changes in behavior.

Change the logic for gitrev as follows

• if the source tree is a git repository, the behavior for build is unchanged. For make dist, append -dist to the git tag in the distributed version of zfs_gitrev.h.
• otherwise, check if the source tree contains zfs_gitrev.h, and use it if so, falling back to "unknown" if it doesn't exist.
• clean it only in make maintainer-clean, so we don't remove it from the source tree on make clean or make distclean.

This allows disted sources to track what git tag they originally came
from, with the -dist suffix indicating that the code wasn't built
directly from git and so might contain additional changes beyond the git
tag.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10595