Remove klist's defname global variable

Remove klist's defname global variable

Addition of a "cleanup" section in kinit's show_ccache() function as
part of commit 6c5471176f5266564fbc8a7e02f03b4b042202f8 introduced a
double-free bug, because defname is a global variable.  After the
first call, successive calls may take place with a dangling pointer in
defname, which will be freed if krb5_cc_get_principal() fails.

Convert "defname" to a local variable initialized at the beginning of
show_ccache().

[ghudson@mit.edu: edited commit message]