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 WRITEFUNC_PAUSE to pause the transfer

Reported By: Zdenek Pavlas
Bug: https://bugzilla.redhat.com/857875
write_abort_test.py: test returning -1 from write callback
setup/teardown consistent with other tests
Correct shebang
Check that bogus return values from write callback are correctly handled (still)
Merge pull request #21 from p-push/write-abort

Write abort support