Add nqp::x_posixerrno
Add test for nqp::x_posixerrno and pir::const::POSIX_*
Add script to generate list of errno value macros
Generate nqp_const.pir as dependency for building NQP
Include nqp_const.pir in registered constants
Ignore generated nqp_const.pir file
Merge pull request #86 from hoelzro/master

Expose errno and POSIX errno values to NQP