util: replace most of SIGALRM blocks with EINTR checks

SIGALRM should not really be an issue for us, most of our IO shouldn't take long (posix_fallocate is the exception) so we can just retry it instead of blocking someone's use of SIGALRM.

Doing it here mostly means that the rest of libei doesn't have to care about EINTR, it'll just be handled transparently (just like before with the SIGALRM blocks).

cc @ofourdan but still needs more real-world testing

Sits on top of !259 (merged) because that one we'll definitely need.