Add pretty-printer for errno.

Add pretty-printer for errno.

This patch adds the actual pretty-printer for errno.  I could have
used Python's built-in errno module to get the symbolic names for the
constants, but it seemed better to do something entirely under our
control, so there's a .pysym file generated from errnos.texi, with a
hook that allows the Hurd to add additional constants.  Then a .py
module is generated from that plus errno.h in the usual manner; many
thanks to the authors of the .pysym mechanism.

There is also a test which verifies that the .py file (not the .pysym
file) covers all of the constants defined in errno.h.

hurd-add-errno-constants.awk has been manually tested, but the
makefile logic that runs it has not been tested.

	* stdlib/errno-printer.py: New pretty-printer.
	* stdlib/test-errno-constants.py: New special test.
	* stdlib/test-errno-printer.c, stdlib/test-errno-printer.py:
	New pretty-printer test.
	* stdlib/make-errno-constants.awk: New script to generate the
	.pysym file needed by errno-printer.py.
	* stdlib/Makefile: Install, run, and test all of the above, as
	appropriate.

	* sysdeps/mach/hurd/hurd-add-errno-constants.awk: New script to
	add Mach/Hurd-specific errno constants to the .pysym file used by
	stdlib/errno-printer.py.
	* sysdeps/mach/hurd/Makefile: Hook hurd-add-errno-constants.awk
	into the generation of that .pysym file.