Merge r1869387, r1869441, r1869456, r1869459, r1869527, r1869533, r1869539, r1869540...

Merge r1869387, r1869441, r1869456, r1869459, r1869527, r1869533, r1869539, r1869540, r1869548, r1869550 from trunk:

Add a basic Travis config.

Pull in more -dev packages to get more modules building (including mod_md).
Test gcc 7 on bionic, xenial is the Travis default anyway.
Only add the toolchain PPA where really needed.

Update travis config

* Add 'if TRAVIS_OS_NAME' to before_install/before_script/script to allow multi-os testing.
* Add separation lines in matrix config (better visual impact).
* Add Linux to all the current job's name.
* Add tests for Worker mpm.

Move before/run scripts out of .travis.yml.

Reviewed by: elukey

Support 2.4.x builds, which lacks --with-test-suite support.
Build with --enable-load-all-modules only when the test suite is run.

Add Travis support for multiple APR/APR-util versions.
Add an APR build with --enable-pool-debug.

Build all modules when testing APR variations.

Build APR-util 1.6.x --with-crypto otherwise the "mod_crypto" build fails,
though this is a bug: https://travis-ci.org/apache/httpd/jobs/609166867

Add APR 1.4/APR-util 1.4 build, but only for 2.4.x.

Start sketching out some travis docs and check we can -> [skip ci]

Submitted by: jorton, elukey

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1869554 13f79535-47bb-0310-9956-ffa450edef68