* trace: Additional include files support

Similarly to `argdist`, `trace` now has a `-I` option for adding
include files that can be used in filter and print expressions.
This also required a slight modification to `argdist`'s syntax
for consistency: where previously we would allow `-I header1 header2`,
we now require `-I header1 -I header2` to avoid any mixups with
which argument is a header file and which is a probe for `trace`.

This is very unlikely to break anyone, because I haven't seen the
`-I` option used at all, not to mention extensively with multiple
headers.

Also made sure the man and example pages are up to date.

* argdist: Update -C and -H switches for consistency

This commit updates `argdist`'s `-H` and `-C` switches for consistency
with the `-I` switch and `trace`'s switches. Specifically, each probe
needs an explicit `-C` or `-H` specifier in front of it. This also
allows safe and understandable mixing of histogram and counting probes,
for example:

```
argdist -C 'p:c:write()' -H 'p::vfs__write(int fd, const void *buf, size_t size):size_t:size#write sizes'
```

* trace: Fix stack trace support for tracepoints

Tracepoint probes don't have a `ctx` argument, it's called `args`
instead. The recently-added stack trace support code didn't take
this into account, and consequently didn't work for tracepoints.
This commit fixes the issue, so we can now do things like
`trace -K t:block:block_rq_complete`.