This also removes LINT_FILE environment variable, other then that functionality 
is kept. It is expected that developers needing partial linting will use `make 
lint`, touching interesting file before (if not done already by writing to 
them).

Lesser form of include-what-you-use: at least guarantees that header
file did not forget to include something through some other included
file.

Activate run_single_includes_tests on CI.
Fix some IWYU violations.

References #5321

Distributions like RHEL7 (yum) install CMake as `cmake3`.

Closes #6163

- Add support for TEST_FILE to the `oldtest` target, for consistency
  with the busted/lua tests.
  Caveat: with the busted/lua tests TEST_FILE takes a full path, whereas
  for `oldtest` it must be "test_foo.res".
- Add support for NVIM_PRG, again so that all test-related targets are
  consistent.
- Use consistent name for NVIM_PRG. But still need to support NVIM_PROG
  for QuickBuild CI.

Note: The `oldtest` target is driven by the top-level Makefile, because
it requires a TTY. CMake 3.2 added a USES_TERMINAL flag to
add_custom_target(). But we support CMake 2.8...

    add_custom_target(oldtest
      COMMAND make clean
      COMMAND make NVIM_PRG=$<TARGET_FILE:nvim> $ENV{MAKEOVERRIDES}
      DEPENDS nvim
      WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/src/nvim/testdir"
      USES_TERMINAL true
    )

This reverts commit c13f72ee0a6600963a10e57155d2e814d56599e2.

References #5447.

We use a Makefile which in turn uses cmake. If we wanted to set the install
prefix for cmake, we had to do this so far:

  make CMAKE_FLAGS="-DCMAKE_INSTALL_PREFIX=/tmp/nvim"

That's long and hard to remember. Following the conventions of other Makefiles,
this now works as well and is equivalent:

  make PREFIX=/tmp/nvim

Running tests from the top-level Makefile will use the third-party
dependencies from .deps instead of the ones from the Travis cache.