Allow named pipes for our log files.

Closes ticket 12061. Based on a patch from "carlo von lynX" on tor-dev at
  https://lists.torproject.org/pipermail/tor-dev/2014-April/006705.html
Fix return value of tor_fd_seekend.

Previously, we had documented it to return -1 or 0, when in fact
lseek returns -1 or the new position in the file.

This is harmless, since we were only checking for negative values
when we used tor_fd_seekend.