Removed GNUNET_CREDENTIAL_Credential, new cmdline parameters, formatting:

Removed GNUNET_CREDENTIAL_Credential, new cmdline parameters, formatting:

- removed all files and functions related to GNUNET_CREDENTIAL_Credential, has been replaced by GNUNET_CREDENTIAL_Delegate
- renamed cmline parameter 'credential'
- added cmline parameter for backward/forward search and a related enum
- added cmline parameter to store private delegates
- only private delegates are used as start/end of the search algorithm (set to vrh in collect)
- run valgrind: no memory exceptions
- run clang-format