pam-util: implement our own pam_prompt() replacement, that doesn't log loudly

pam-util: implement our own pam_prompt() replacement, that doesn't log loudly

pam_prompt() will log very noisely at high error levels if it is called
without a conversation function that works. This is however a frequent
case, given that ssh doesn't provide one. To tone down the misleading
logging a bit, implement our own pam_prompt_graceful() that is just like
pam_prompt(), but reports errors back the caller who then logs (which we
generally do anyway).