os_fchown: impl and remove HAVE_FCHOWN
os_fchown: add unit tests
TEMP: 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.