fix: Update xsnap/xsnap-worker argument processing and usage output

[gibson042]

Builds on #51

The following xsnap/xsnap-worker discrepancies are implicitly documented but not addressed:

• -d <snapshot> (read heap snapshot file and dump a textual representation to stderr) is not supported by xsnap-worker.
• The option to prefix print(…) output with the current meter count in square brackets is -q for xsnap but -p for xsnap-worker.
• xsnap supports -s (interpret each non-option operand as the path for a script to load), while xsnap-worker supports -s <size> (set parser buffer size, in kiB (defaults to 8192 = 8_388_608 bytes)).

[phoddie]

This all looks reasonable.

(Fair warning: I read the docs and code but didn't confirm things work as described by building and running.)