Cleanup, additional input checks, renaming, simplification:

Cleanup, additional input checks, renaming, simplification:

- introducing own GNUNET_SIGNATURE_PURPOSE_DELEGATE
- renaming of cred/crd in delegation_misc.c
- renamed extension cmd para to import
- changed subject key/attr parsing from memcpy/malloc to strtok
- only allow to create delegates to expire absolute not relative (prevent reusing created delegation signatures)
- check subject key and reuse expiration of import/signed delegation
- replaced strdup() part of delegation_misc.c and credential_serialization.c with pointers
- uncommented return after detection of unverifyable signatures