KCM: Document KCM debugging

[aplopez]

Added some KCM-specific details to the already present debug information.

[justin-stephenson]

I added some inline comments but changes look good to me overall.

A couple other things I found useful when debugging KCM issues. You can determine if they would be worthwhile to add here or not.

• KCM opcodes and parameters from https://github.com/krb5/krb5/blob/master/src/include/kcm.h
• Strace command to dump of the KCM socket data (protocol request/response), intermixed with sssd KCM debug logging:

sudo strace -p $(pidof sssd_kcm) -Ttxvs 1024 -e read=$fds -e read,readv,write

[aplopez]

* KCM opcodes and parameters from https://github.com/krb5/krb5/blob/master/src/include/kcm.h

I didn't add this one as it is easily found and the text mainly focuses on debugging techniques.

sudo strace -p $(pidof sssd_kcm) -Ttxvs 1024 -e read=$fds -e read,readv,write

But I did add this one.

Thanks.

[justin-stephenson]

Ack, thank you.