Add codecheck make target

Add codecheck make target

We'd like to have tooling that verifies code style, while ignoring the
commit message.  For example, code does not need to be signed off in
order to be tested.  Current workarounds are to run `git checkstyle` and
ignore the commit message errors, or to run `make cstyle shellcheck
flake8 mancheck testscheck`, and make sure that list stays updated.

Solution is to add a new make target, `codecheck` which does all the
code checks.  `checkstyle` is now simply `codecheck` + `commitcheck`.

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #7985