Merge r1869541, r1869543, r1869564, r1869565, r1869684, r1869697, r1869703, r1869708...

Merge r1869541, r1869543, r1869564, r1869565, r1869684, r1869697, r1869703, r1869708, r1869729, r1869801, r1869846, r1870047, r1870077, r1870080, r1870086, r1870150, r1870550 from trunk:

Run buildconf using the specific APR/APR-util versions if configured.

Revert r1869541 and always run httpd's buildconf against the Debian APR,
which does include the *.m4 files.

Add shared-mpms build, add build against APR trunk (httpd trunk only).
Fix handling and ensure safe caching of APR trunk build.

Fix yaml typo.

* .travis.yml: Build on Bionic by default, only build w/APR 1.5.x for
  the 2.4.x branch.

* .travis.yml, test/travis_run_linux.sh: Pass BUILDCONF to buildconf
  and add job which uses the new option.

Install PCRE v2 (10.x) by default, add a job testing with PCRE "v1" (8.x).

Minor Travis doc updates and a todo list.

Test IRC and e-mail notifications.

Add build job for trunk on Xenial with custom APR/APR-util builds.

Try mailing dev@ for Travis build results.

The defaults are to mail:
- on success (if changed from failure)
- on failure (always)

Specify make flags globally and work around PR 63942 for regeneration
of server/util_expr parser.

Support travis builds against APR/APR-util non-trunk branches.
Add job for APR 1.7.x + APR-util 1.7.x.

Build on s390x (interesting test target since it's big-endian).
IRC notifications work, e-mail doesn't; branches are done.
ppc64le builds also work but are extremely slow, so not enabled.

Doc updates for Travis.

Note that APU_VERSION=trunk doesn't work per rpluem's mail.

Skip ap_expr regeneration for 2.4.x.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1870551 13f79535-47bb-0310-9956-ffa450edef68