Fix clippy diagnostics

[squell]

This is best reviewed commit by commit: first commit handles cargo clippy warnings, the second fixes cargo clippy --test warning.

In the first commit, some of the clippy ignores are unavoidable; in the second, it's test code so we don't really care about the "large Error case" warning.

There were also some correct nitpicks by clippy (the ommission of truncate and the location of the the make_zeroed_sigaction function)