unix: add SEEK_* constants on Darwin, FreeBSD, Linux

The Go "os" package already provides bindings for SEEK_CUR, SEEK_SET and
SEEK_END. Most operating systems also support SEEK_HOLE and SEEK_DATA,
which you can use to skip sparse regions in a file. Let's add bindings,
so we can also do this from within Go.

Change-Id: If9243b05a8f563b4bce2452aa4bff145d9442cc2
GitHub-Last-Rev: ac8aed2
GitHub-Pull-Request: #112
Reviewed-on: https://go-review.googlesource.com/c/sys/+/328170
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

Author: EdSchouten

unix/mkerrors.sh

@@ -563,6 +563,7 @@ ccflags="$@"
 		$2 ~ /^KEYCTL_/ ||
 		$2 ~ /^PERF_/ ||
 		$2 ~ /^SECCOMP_MODE_/ ||
+		$2 ~ /^SEEK_/ ||
 		$2 ~ /^SPLICE_/ ||
 		$2 ~ /^SYNC_FILE_RANGE_/ ||
 		$2 !~ /^AUDIT_RECORD_MAGIC/ &&