Add argument checking to sigemptyset(3) and friends.

Add argument checking to sigemptyset(3) and friends.

You could argue that this is hurting people smart enough to have manually
allocated a large-enough sigset_t, but those people are smart enough to
implement their own sigset functions too.

I wonder whether our least unpleasant way out of our self-inflicted 32-bit
cesspool is to have equivalents of _FILE_OFFSET_BITS such as _SIGSET_T_BITS,
so calling code could opt in? You'd have to be careful passing sigset_t
arguments between code compiled with different options.

Bug: 5828899
Change-Id: I0ae60ee8544835b069a2b20568f38ec142e0737b