frontend: Always reset the current command after handling it.

That is, also reset it after an authentication error.
frontend: Always store errors in the error buffer.

This allows to access the error from other parts of the code.
strbuf_test: Added some more test-cases for sdb_strbuf_skip().
utils strbuf: Don't copy more than needed in sdb_strbuf_skip().
frontend: Clear the error buffer when initializing a new command.
frontend: Reset the current command length on CONNECTION_IDLE.
connection_test: Added first test for frontend connection handling.

For now, testing a simple session startup.