Trace plugin module loading errors

Add trace messages when dlopen() or dlsym() fails when loading a
plugin module.

https://github.com/krb5/krb5/commit/e362c356eafb49a9d90a4f20c6668682d4f50222
Author: Greg Hudson <ghudson@mit.edu>
Commit: e362c356eafb49a9d90a4f20c6668682d4f50222
Branch: master
 src/include/k5-trace.h    |    5 +++++
 src/lib/krb5/krb/plugin.c |   14 ++++++++++++--
 2 files changed, 17 insertions(+), 2 deletions(-)