[server] Call clean_exit() from daemon parent process

When becoming a daemon, make sure the fwknopd parent process calls
clean_exit() to release memory before calling exit().
[server] add access.conf variable GPG_FINGERPRINT_ID

Add a new GPG_FINGERPRINT_ID variable to the access.conf file
so that full GnuPG fingerprints can be required for incoming SPA packets
in addition to the appreviated GnuPG signatures listed in GPG_REMOTE_ID.
From the test suite, an example fingerprint is

GPG_FINGERPRINT_ID            00CC95F05BC146B6AC4038C9E36F443C6A3FAD56
[server] Require sig ID's or fingerprints when sigs are validated

When validating access.conf stanzas make sure that one of
GPG_REMOTE_ID or GPG_FINGERPRINT_ID is specified whenever GnuPG
signatures are to be verified for incoming SPA packets. Signature
verification is the default, and can only be disabled with
GPG_DISABLE_SIG but this is NOT recommended.