Fix leash crash found in some build environments

Fix leash crash found in some build environments

When freeing a credentials cache name obtained from
krb5_cc_get_full_name(), the code was using plain free()
instead of the matching krb5_free_string().  If these routines
are picked from different modules at runtime, the mismatch
will cause a crash in free(), so change to using the matched
deallocation function.

In order to use it in leash, it must be declared in Lglobals.h and
the function pointer symbol defined in Leash.cpp.