[krbdev.mit.edu #8011] Problem regarding 'Pass phrase' prompt length limit in pkinit plugins

[Greg Hudson via RT]

prompt_string is currently a stack-allocated array, so it has to be a fixed 
length.  The length can't be all that big, since threads can have 
constrained stack sizes in some circumstances.

We should remove this limitation by changing prompt_string to a pointer and 
using asprintf instead of snprintf.