Make plugin auto-registration work on Windows

Make plugin_base_dir subject to parameter expansion, and set its
default to %{LIBDIR}\plugins on Windows (%{LIBDIR} expands to the
directory where the krb5 DLL lives).  For auto-registered modules,
assume that we will build 32-bit and 64-bit DLLs and name them
"modname32.dll" and "modname64.dll".  In k5_plugin_register_dyn(), use
k5_path_join() instead of assuming the path separator is "/".

https://github.com/krb5/krb5/commit/bae5aae9a6e937e4eaa995e6a0d68b398199d841
Author: Greg Hudson <ghudson@mit.edu>
Commit: bae5aae9a6e937e4eaa995e6a0d68b398199d841
Branch: master
 doc/admin/conf_files/krb5_conf.rst |    3 ++-
 src/include/osconf.hin             |   11 +++++++++++
 src/lib/krb5/krb/init_ctx.c        |    9 ++++++---
 src/lib/krb5/krb/plugin.c          |    8 ++++++--
 4 files changed, 25 insertions(+), 6 deletions(-)