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 <[email protected]>
Reviewed-by: Eli Schwartz <[email protected]>
Signed-off-by: Arvind Sankar <[email protected]>
Closes #10595

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