The caller never uses the same allocated memory
for the xseg context again. Instead calls xseg_join()
to rejoin the segment. This introduces a memory leak every time
the caller leaves and rejoins the shared memory segment on the
same process address space.

* 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.

Various libxeg improvements

Remove umask calls and make fifo mode 666. This way, we avoid calling umask from
the librar and we rely on the umask of each process in order to create the pipes
with the right permissions.

Propagate the correct errno to the caller of xseg_join function.

Remove 'posixfd' segment type. This segment type was identical to 'posix' type
and posed no added benefit. Moreover, it was never used by any xseg or
Archipelago componenent.

Propagate errno when calling an operation on the 'posix' segment type.

Replace getting the current program counter from labels, since it may break with
different optimization settings. Since the previous implementations could be
greatly influenced by the compiler's behavior, switch to an assembly-based
approach.

Do not implicitly create posixfd directory. Instead, expect it to be already
setup correctly and return an error otherwise.

Add S_ISGID bit to fifo's directory. This way each fifo created under this
directory inherits the group id of the POSIXFD directory. This enables different
Archipelago components to signal each other independently from their effective
user id.

Make sure that the new logfiles are readable and writable by the group of the
process that creates them.

Make sure the pipes that are created, are readable/writable by the group of the
process that creates them.

Improve xlock