PR12749: Replace popen calls with stap_spawn_piped

PR12749: Replace popen calls with stap_spawn_piped

The new form has the advantages that child processes are managed by
signals to stap, and that arguments are provided in a vector so they
don't need to be escaped.

* dwflpp.cxx (dwflpp::iterate_over_libraries): Convert popen call to
  stap_spawn_piped, followed by fdopen so the same FILE* operations are
  still supported.  Finish with fclose+stap_waitpid instead of pclose.
* tapsets.cxx (symbol_table::read_from_elf_file): Ditto.