[WIP] Native implementation for USDT probes

To avoid exposing the implementation of new/delete of the Argument
class, move it out of the header file. This also requires making private
the std::string members, so that they cannot be assigned to directly.
Let the ArgumentParser assign to them as a friend, and expose const
functions to access them as const only from the client.

Also, convert non-const reference usages to pass-by-pointer.

bpf: Wrap `bcc_procutils_which_so` as `BPF.find_library`

This class method was being used outside of the BPF class in the
`tracer` and `usdt` tools.

Explicitly list the libelf runtime dependency

Add libelf-dev to Ubuntu build deps

This was required on a Debian system, but in lieu of instructions for that distribution, and as Debian users will follow Ubuntu, would you consider verifying it on Ubuntu and accepting this PR?

[RFC] Remove all dependencies on external binaries

The native resolvers are used for the Kernel Symbol resolution in the
`BPF` class, and for the ProcessSymbols class. A lot of redundant code
has been removed.

The `sym` module no longer needs to be exported. The `LD` module is no
longer used anywhere.

The static `BPF.SymbolCache` now uses a native symbol resolver instead
of the Lua/binutils implementation. Likewise for the kernel symbol
resolver, and the `check_path_symbol` API to find a probe's hook
address.

This new set of C/C++ APIs is capable of performing symbol resolution
for any given running process, using the native `libelf` APIs instead of
shelling out to binutils tools.

This is a reimplementation of the original ProcSyms Python/Lua code.

These APIs mimic behavior that was being performed using native tools.

- bcc_procutils_which_so: resolves a short library name into a full path
  so a shared object. Equivalent to `ldconfig -p`
- bcc_procutils_which: resolves a short name into a full path to a
  binary. Equivalent to a shell's builtin `which`
- bcc_procutils_each_module: lists all the modules loaded in a process'
  memory space. Uses the kernel's `/proc/$pid/maps`
- bcc_procutils_each_ksym: lists all symbols and addresses for the
  kernel. Uses the kernel's `/proc/kallsyms`

The following APIs have been implemented inside `libbcc`, which
programatically replicate the previous usage of `binutils` tools like
`objdump` and `elfdump`.

- bcc_elf_foreach_usdt: finds all USDT probes inside a given binary
- bcc_elf_loadadddr: find the load address for a shared object
- bcc_elf_foreach_sym: lists all symbols in an ELF binary or SO
- bcc_elf_shared_obj: returns whether a given path is an ELF shared
  object, a binary, or neither

Add a new a `FindLibElf.cmake` package, and use it to link the main
`libbcc` library against the system's LibELF. This library will be used
to re-implement functionality that was previously dependent on
`binutils`.

Instead of importing the `Find*.cmake` modules directly, place them in
the root `cmake/` folder and define it as a CMake Module path. This lets
us use the `find_package` builtin to require any given packages.