Correct grammer in MAINTAINERS.md

Signed-off-by: Stephen Crosby <stevecrozz@gmail.com>
Add a "daemon" build tag and toggle it with the already-existing "DOCKER_CLIENTONLY" build variable

This works mostly by refactoring our "main" package to be careful about what it imports based on the daemon build tag. :)

Also, I've updated Travis to test "client-only" compilation after it tests the daemon version.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Adjust build tags to be client-only in "make cross" too

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #7310 from stevecrozz/maintainers-grammar-update

Correct grammar in MAINTAINERS.md
Merge pull request #7377 from tianon/daemon-build-tag

Add a "daemon" build tag and toggle it with the already-existing "DOCKER_CLIENTONLY" build variable
Rename DOCKER_CONFIG envvar to DOCKER_CERT_PATH

DOCKER_CONFIG was introduced in #6984.

We may use "config" for other purposes (e.g. #7232). Until we
have made a design decision around how configuration files will
work, DOCKER_CERT_PATH is a much safer name to rely on for future
compatibility.

Docker-DCO-1.1-Signed-off-by: Ben Firshman <ben@firshman.co.uk> (github: bfirsh)