Fix PKCS11 module path search

Fix PKCS11 module path search

Commit c5c11839e02c7993eb78f2c94c75c10cf93f2195 switched the loading
of the PKCS#11 module from dlopen() to krb5int_open_plugin().  Because
krb5int_open_plugin() includes a stat() test, this change has the
unintended consequence of requiring the module name to be an absolute
or relative path to the library, not a filename within the dynamic
linker search path.

Within krb5int_open_plugin(), only stat() the filename on the
platforms which will use the file type.

[ghudson@mit.edu: adjusted conditionals to call stat() on Windows;
rewrote commit message]

(cherry picked from commit e134d9a6b6332bd085093e9075c949ece784fcd0)

ticket: 9067
version_fixed: 1.20.1