os_fchown: impl and remove HAVE_FCHOWN
os_fchown: add unit tests
os_fchown: enable 'change group' unittest on Travis-CI

Add section `before_install` in `.travis.yml` to create test group and
add current user to this group.
It is needed because by default user on Travis-CI belongs only to one
primary group derived from that user. So we have no alternative to
change group of the file.
Travis-CI: fix "Could not find compiler" in clang-asan
os_fchown: remove cast of parameter -1 to uid_t
Merge pull request #950 from Hinidu/os_fchown

Implement os_fchown and remove HAVE_FCHOWN
Add a release build to the test matrix.
Avoid an unused variable warning in the release build.
Add a release build for gcc.