pycurl.c: allow to return -1 from write callback
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

p/pycurl-archived,p/pycurl-archived,pycurl/pycurl,p/pycurl-archived,pycurl/pycurl,pycurl/pycurl