return a pointer to the end of the valid part of the sigstack, no further
return a pointer to the end of the valid part of the sigstack, no further

also unmap the whole thing when cleaning up, rather than leaving a spare
page floating around.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust