Verify .gitignore entries

This change adds a make target 'vcscheck' which scans the git workspace
for new, untracked files missing from the .gitignore configuration; this
is done to help prevent adding unwanted build artifacts to the source
tree during development.

Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8281