Driver: Check for PROT_EXEC on translation, gracefully handle SIGBUS

[skmp]

Currently we'll happily translate pages without PROT_EXEC or not mapped at all.

We need to only permit code to run in guest mapped pages, and only ones with PROT_EXEC, or appropriately generate SIGSEGVs for the guest.

We also need to gracefully handle SIGBUSes during translation, eg from PROT_EXEC mappings that are out of range.

We also need FLTs for those.