Call sigsetjmp/siglongjmp directly in code

[jpco]

This lets us control when we do want to save the signal mask (i.e., when longjmping out of the signal handler) or don't (i.e., whenever we're just doing exceptions).

It's a bit more explicit, which is nice, given the "function X is actually slightly-different function Y" macrology can be confusing (I've been bitten by it in the past).

There's also a minor but possibly non-negligible performance benefit, since it makes the most common case, setjmp calls as part of ExceptionHandler macros, ~100x faster by cutting out a couple syscalls for each setjmp(). This is a relatively tiny benefit, but given ExceptionHandler is called for every $&parse, $&here, redirection, lambda evaluation, every time a dynamically-scoped variable is popped, twice for each $&catch, and more, it seems reasonable to make this pretty tiny and simple optimization. If #73 or something similar ever gets merged such that if and <= also catch certain exceptions, then this would help even more.