memdebug improvements

[bagder]

I bring two changes in this set:

1. the memdebug system now also covers send, recv and socket properly. They're logged and they're affected by the "memlimit" system that's used by the torture tests which means that by setting the correct environment variable we can make those calls return failures as well. runtests.pl -t that covers more exit paths for each test.
2. Starting now, valgrind is used for the torture tests as well if found and not disabled. Previously, the torture tests only used the internal memory debug system which is disabled when libcurl is built with the threaded resolver! Valgrind of course detects leaks and memory problems even better and with better error reporting. Using valgrind on torture tests makes them MUCH slower so beware.