This fixes the compile with `-stdlib=libc++`. Without this header the build fails with:

```
../src/cc/ns_guard.cc:75:30: error: use of undeclared identifier 'CLONE_NEWNS'
```

Currently, if more than one pid-associated USDT attaching to
the same probe, usdt readarg code will be generated twice and
the compiler will complain.

This patch solves issue by preventing code duplication if
a previous context with the same mnt point and exec binary
has generated the code for the same probe. The event name is
also changed to have pid embedded so different pid-associated
uprobe event will have different names.

This patch introduces an internal uprobe event name
discrepency. It is a good idea to have event name
generation in libbpf so that both C++ API and Python API
will have consistent name conventions. This will be
addressed in a subsequent commit as it is largely
a different issue.
Signed-off-by: Yonghong Song <yhs@fb.com>

This moves ProcMountNS[Guard] classes to a separate header file and
implementation. The goal will be to use these more directly in libbpf.c
without creating a circular dependency.
Removed the friend class statements and exposed a few public getters instead.
Slightly changed the implementation in ProcMountNS constructor to avoid
a few error cases and the use of snprintf.
Signed-off-by: Brenden Blanco <bblanco@gmail.com>