PR10485: auto-path tapset support

PR10485: auto-path tapset support

This prefixes the process probes placed in the tapset/PATH/ directory
with the location of the tapset.

* main.cxx: New collect_{stp,stpm} functions for passing to nftw(). Parse all
  .stp files placed in include_path[0]/PATH/. nftw() is now used instead of
  globbing for tapset searching to allow recursive traversal.
* parse.h: New pf_auto_path flag.
* parse.cxx: Prefix process argument with location of tapset.