After verifying that they really were spurious, clean up the warnings
that gcc -wextra reports, except for LeaMDNS.
Upgrade GCC to gcc-7 for host builds
* Add full debug builds for all tests
In order to ensure the -DEBUG builds don't fail with checked-in code, add
two jobs which build even/odd with full debug flags to Travis.
Fixes#5143
* Add in dump of platform.txt to verify build options
* Fix BASH comparison of strings
* Fix debug-mode compile error in ESP8266AVRISP
* Add -Wextra to CI (matches IDE "All" warnings)
Use platform.local.txt to add -Werror to GCC for the build of all
code. Any warnings on a submitted patch will cause an error.
Several examples and libraries had warnings/errors (missing returns
on functions, types, etc.). Clean those up with this commit as well.
