Add support for "{ptype}" trace format specifier

Add the "{ptype}" trace format specifier, for principal name types.
Also document the new option in the "k5-trace.h" comments.

https://github.com/krb5/krb5/commit/27417b7d27bd61acf567da2ddccc591b7f499e47
Author: W. Trevor King <wking@tremily.us>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 27417b7d27bd61acf567da2ddccc591b7f499e47
Branch: master
 src/include/k5-trace.h  |    1 +
 src/lib/krb5/os/trace.c |   24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+), 0 deletions(-)