test_internals.py: temporarily restored to fix 'make test'
pycurl.c: eliminate duplicated code in util_write_callback()

Suggested by Zdenek Pavlas <https://bugzilla.redhat.com/857875#c8>.
pycurl.c: allow to return -1 from write callback

... to abort the transfer and CURL_WRITEFUNC_PAUSE to pause the transfer

Reported By: Zdenek Pavlas
Bug: https://bugzilla.redhat.com/857875
test_write_abort.py: test returning -1 from write callback