Small KBUS utility

Just committed errno.py to kbus/utils - this is a simple utility to conver Unix error numbers into their corresponding mnemonic and text, or vice versa:

 $ errno.py EPERM
EPERM is error 1 (0x1): Operation not permitted

but also, if the error is one used by KBUS, to give the appropriate text from the KBUS documentation:

$ errno.py 32
Error 32 (0x20) is EPIPE: Broken pipe

KBUS:
On attempting to send *to* a specific replier, the replier with that id is no
longer bound to the given message's name.

because I can never remember which magic number corresponds to what particular usage.