fd: Improve error handling of write_nonblock().

fd: Improve error handling of write_nonblock().

This function had two shortcomings: First, a call to write might
fail with errno set to EINTR in case the write call was interrupted
before any data was written. This is not fatal and one should just
retry the write in this case.

Secondly, POSIX allows to return either EAGAIN or EWOULDBLOCK if the
write would block but we only check for EAGAIN. This is no problem on
Linux since both constants refer to the same value on Linux. However,
POSIX does not require them to be equal, so we have to check for both.

This patch corrects both issues.