Explicitly state supported Linux versions

Add META tags Linux-Maximum and Linux-Minimum.

One pain point for package maintainers is ensuring the compatibility of
the packaged version of ZFS with the Linux kernel. By providing an
authoritative compatibility guide in the source tree, maintainers can
automate compatibility checks.

Additionally, increase META string extraction specificity.
configure.ac finds Name and Version by a very simple `grep`, which might
conceivably find other fields. Require the string be at the beginning of
a line, and be followed by a colon to avoid such confusions.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #7571

META

@@ -1,8 +1,10 @@
-Meta:         1
-Name:         zfs
-Branch:       1.0
-Version:      0.7.0
-Release:      1
-Release-Tags: relext
-License:      CDDL
-Author:       OpenZFS on Linux
+Meta:          1
+Name:          zfs
+Branch:        1.0
+Version:       0.7.0
+Release:       1
+Release-Tags:  relext
+License:       CDDL
+Author:        OpenZFS on Linux
+Linux-Maximum: 4.16
+Linux-Minimum: 2.6.32