Use errno module instead of hardcoding EINPROGRESS
Use errno module instead of hardcoding EINPROGRESS

FreeBSD uses a different value for this errno than Linux, which caused
incorrect handling of TCP connections.  This could result in UDP being
used for scenarios where it was too large for the KDC, causing all
requests to fail.

[rharwood@redhat.com: rewrote commit message]
Merges: #50

latchset/kdcproxy,npmccallum/kdcproxy