[krbdev.mit.edu #5749] KFWLOGON: FEATURE: Support FILE ccaches as configured for NIM

[Jeffrey Altman via RT]

FNAL Help Desk requests that the kfwlogon integrated logon functionality
store credentials not into the API: ccache but into a FILE: ccache as
specified in the NIM Identity configuration.  They use FILE: ccaches
instead of API: ccaches in order to permit the credentials to be used by
cygwin apps and Java apps in addition to KFW apps.