RFC: access to sigatomic_begin/end in Julia for non-interrupt-safe C calls

[stevengj]

See also issue #2622. This adds Julia-callable sigatomic_begin/end functions to temporarily defer SIGINT (ctrl-c) handling, which is essential for long-running C calls that are not interrupt safe. These are wrapped in a higher-level disable_sigint() do ... end construct. Moreover, it also allows one to re-enable SIGINT handling within a disabled region, e.g. for Julia callbacks that can catch the exception.

(This will enable my PyCall module, for example, to defer interrupts during Python calls, which are not interrupt safe. But if the Python has a Julia callback, that Julia callback can still catch InterruptExceptions, convert them to Python KeyboardInterrupt exceptions which are propagated back through Python and returned to the top-level Julia caller, which converts it back to an InterruptException.)

As mentioned in #2622, in the long run we will almost certainly want to make sigatomic the default for all ccall invocations (since C code cannot generally be assumed to be interrupt-safe), and if this is done directly in the LLVM code generation it can be much more efficient than calling disable_sigint from Julia. However, even when that is implemented, we will still want "manual" access to sigatomic_begin/end in Julia in order to re-enable interrupts in callbacks as I mentioned above.