Fix S2ECallBuilder, make it as it was before

The idea is to return the CallInst even if it's already inserted to the
code. The caller may modify a few attributes that way.
Rename pass to BBMarker

The goal of this whole thing is to write a pass that facilitates the
enumeration and instrumentation of basic blocks in Chef, I guess.
Add method to easily insert complex S2E opcodes

This is a more similar way to the guest level API headers that expect an
operand and (optionally) a parameter. Here we additionally accept
(optionally) an 2-byte argument in order to make full use of the
available 4 bytes.

The format of the opcode will be:

	Arg (higher 8 bits) | Op | Param | Arg (lower 8 bits)

This should generate something like

	0x00, 0xBB, 0x01, 0x??

As long as Argument<256, and we can keep it sane enough.