Fix various spelling errors

[github at samba.org]

New comment by sathieu on Samba Github repository

https://github.com/samba-team/samba/pull/13#issuecomment-142784949
Comment:
Those are mostly harmless (comments or error strings).

Still, carefully review the following:
* source3/include/MacExtensions.h : should the typo be kept?
* source4/heimdal/lib/asn1/rfc2459.asn1, even if the typo is not in the RFC