This fixes a possible file table leak if a filetable contains an fd
referring (strongly) to the filetable itself. Now, it will automatically
be downgraded to a weak reference after it becomes a context's active
file table.

TODO: maybe support consuming a context's filetable to get a strong
reference back, provided it's exclusively owned.

This fixes a very rare (requires that an altreloc span two pages) kernel
crash where it writes past the region that was remapped as RWX, thus
triggering an in-kernel "wrote without write access" page fault.

Additionally, this fixes the logic for unavailable features. It fixes
the short-circuit that would forget to remap the memory back from RWX to
R-X. NOP bytes are also now replaced with multi-byte NOP regardless of
whether the CPU supports the given feature.

This fixes a Cargo deadlock.

Only the allocator is allowed to switch between 0 and 1. Refcounts above
1 are however controlled by the virtual memory system.