examples: Simplify signal handler

[danielgustafsson]

The signal handler registered for SIGINT is only handling SIGINT
so there isn't much need for inspecting the signo. While there,
rename the handler to be more specific.

g_should_exit should really be of sig_atomic_t type, but relying
on autoconf in the examples seems like a bad idea so keep tjat
for now.

Closes #xxxx

[dfandrich]

I've just discovered that sig_atomic_t is part of C89, so you should be able to use that type here unconditionally. Examples aren't intended to be as fully portable as the rest of curl, anyway. It also means that I can just submit PR#7309 because there's no need to check if that type is available because it will be.

[dfandrich]

IIRC, sig_atomic_t should be available just about everywhere that curl runs, which I discovered when looking at #7309 which means there should be no need for an autoconf test. But don't quote me on that, because I stopped looking further when I discovered it simply isn't used in curl.