silc-client: handle prompt abort better

When verfying public key abort any previously ungoing prompt so that we
can get the public key verification prompt up.  It's important to not
loose the public key verification prompts so we now allow new prompt to
come up after previous one has aborted.  It leaks Irssi memory, but
handles things correctly towards silcclient library.  Irssi's prompt
handling is broken because it stores the data in a global variable
allowing only one prompt at a time.