[fud2] Add consistent parameter to correctly emulate or execute on Xilinx.

[nathanielnrn]

More small parts of #2267. This should fix treatment of the XCL_EMULATION_MODE environment variable, with respect to hw and hw_emu values (aka execution and emulation).
Currently fud2 does not allow for sw_emu values because software emulation requires C files, not something we've really been dealing with.

From some spot checking on havarti seems like hw_emu can compile to an xclbin fine, but targeting actual execution seems to fail? At least on my toy dyn-mem-vec-add.futil file.

Also finally wrote down the explicit steps to use (and the parameters required at each step) and added them to the docs.
I'll move them and polish them up if needed.
To ease dealing with parameters would it make sense to require the [xilinx] parameters to be part of the fud2 config file? That way most of the logic between runs would be consistent, independent of the user remembering what they compiled to originally.