xseg_posix: Propagate errno of segment ops

Propagate errno when calling an operation on the 'posix' segment type.
xseg_posixfd: Remove 'posixfd' segment type

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.
xseg: Propagate errno for xseg_join

Propagate the correct errno to the caller of xseg_join function.