Add a tor_asprintf() function, and use it in a couple of places.

Add a tor_asprintf() function, and use it in a couple of places.

asprintf() is a GNU extension that some BSDs have picked up: it does a printf
into a newly allocated chunk of RAM.

Our tor_asprintf() differs from standard asprintf() in that:
  - Like our other malloc functions, it asserts on OOM.
  - It works on windows.
  - It always sets its return-field.