[krbdev.mit.edu #5735] NIM: FEATURE: LIB: Add API to issue commands to plug-ins from command-line

[Jeffrey Altman via RT]

For example, to ask the kca provider to obtain a cert
from a specific realm using a specified identity.  

netidmgr --provider kcacred --identity jaltman at ATHENA.MIT.EDU --realm
ANDREW.CMU.EDU --getcert