Make rekillable consistent with unkillable

As for now, rekillable is an unsafe function, instead, it should behave
just like unkillable by encapsulating unsafe code within an unsafe
block.

This patch does that and removes unsafe blocks that were encapsulating
rekillable calls throughout rust's libs.

Fixes #8232
Testing rekillable fails when called from outside an unkillable block
Don't make the runtime exit on illegal calls
Decrement unkillable counter before failing
Rebased and replaced yield with deschedule
auto merge of #8581 : FlaPer87/rust/issue/8232, r=bblum

As for now, rekillable is an unsafe function, instead, it should behave
just like unkillable by encapsulating unsafe code within an unsafe
block.

This patch does that and removes unsafe blocks that were encapsulating
rekillable calls throughout rust's libs.

Fixes #8232