ask for more warnings during a Travis CI build

.travis.yml: add 'touch .devel configure'
Coverity: add the build script used by Travis-CI and Coverity Scan

from https://scan.coverity.com/scripts/travisci_build_coverity_scan.sh
Coverity: customize the build script

- add a 'Verify Coverity Scan run condition' step to avoid multiple runs wit
  Travis matrix.
- add a 'Verify Coverity Scan script test mode' step. if true no uploading,
  avoid reaching the quota. usual processing: false.
- send 'description' as VERSION#SHA (e.g.: 1.6.0-PRE-GIT#c661f8b)
Coverity: update .travis.yml for analysis by pushing in 'coverity_scan' branch
In activate routines, don't pre-set the status; set it for all errors.

This fixes some cases where it wasn't being set properly or at all, and
also means that, as it's not being set initially, compilers that do any
dataflow analysis and report uninitialized variables, and static
analyzers, should report cases where we aren't setting variables correctly.

See GH #352.
Always return 0 on a successful activation.
Merge remote-tracking branch 'bpf/master'

bpf.tcpdump.org and GitHub have diverged again.