Refactoring: bytecode interpreter ABI

Refactor the ABI of the bytecode interpreter, which is used by the probe
provider callbacks:

- Return a "int" rather than uint64_t,
- The return values are now:
       LTTNG_UST_BYTECODE_INTERPRETER_ERROR = -1,
       LTTNG_UST_BYTECODE_INTERPRETER_OK = 0,
- Introduce a bytecode "context". This context is specific for each
  bytecode "class" (filter or capture). The filter class context
  has a "result" (accept/reject) output, whereas the capture class
  has an "output".
- The bytecode context is extensible with the struct_size scheme.
- Merge filter and capture interpreters into a single callback.
  That callback uses a new "bytecode type" field within the bytecode
  private data to figure out the specific bytecode class (filter or
  capture).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Id528e141d0c213e9bb41f68d6d7e632104daa1cf