during debug of production systems it's difficult to trace back
the kernel reported 'bpf_prog_4985bb0bd6c69631' symbols to the source code
of the program, hence teach bcc to store the main function source
in the /var/tmp/bcc/bpf_prog_4985bb0bd6c69631/ directory.

This program tag is stable. Every time the script is called the tag
will be the same unless source code of the program changes.
During active development of bcc scripts the /var/tmp/bcc/ dir can
get a bunch of stale tags. The users have to trim that dir manually.

Python scripts can be modified to use this feature too, but probably
need to be gated by the flag. For c++ api I think it makes sense
to store the source code always, since the cost is minimal and
c++ api is used by long running services.

Example:
$ ./examples/cpp/LLCStat
$ ls -l /var/tmp/bcc/bpf_prog_4985bb0bd6c69631/
total 16
-rw-r--r--. 1 root root 226 Sep  1 17:30 on_cache_miss.c
-rw-r--r--. 1 root root 487 Sep  1 17:30 on_cache_miss.rewritten.c
-rw-r--r--. 1 root root 224 Sep  1 17:30 on_cache_ref.c
-rw-r--r--. 1 root root 484 Sep  1 17:30 on_cache_ref.rewritten.c

Note that there are two .c files there, since two different
bpf programs have exactly the same bytecode hence same prog_tag.

$ cat /var/tmp/bcc/bpf_prog_4985bb0bd6c69631/on_cache_miss.c
int on_cache_miss(struct bpf_perf_event_data *ctx) {
    struct event_t key = {};
    get_key(&key);

    u64 zero = 0, *val;
    val = miss_count.lookup_or_init(&key, &zero);
...
Signed-off-by: Alexei Starovoitov <ast@fb.com>