xseg_posixfd: Refactor FIFO creation & pipe ops

* Remove retry loop in FIFO creation. If FIFO already exist's
  do not unlink and re-create the named pipe. Also note that
  mkfifo() is thread-safe and async-cancel-safe.

* As it happens with pipes, writing to a FIFO with no connected
  readers will fail with EPIPE and also deliver a SIGPIPE signal
  to the writer. Reading from a FIFO with no connected writers will
  return an EOF to the reader. Setting the O_RDWR flag while opening
  the pipe will create an equal number of connected readers/writers
  to the kernel and will ensure that both are above zero.
  This implementation will NEVER block since the process can at least
  talk to itself.
  ** POSIX.1 leaves this case "undefined" when O_NONBLOCK is set. **

* Drain the pipe even if the call was interrupted by a signal
  before any data was read. Optimize drain loop to make the
  minimum number of read syscall's.

* Write to the pipe even if the call was interrupted by a signal
  before any data was written.