fix const-correctness in sigandset/sigorset arguments

this change is consistent with the corresponding glibc functions and
is semantically const-correct. the incorrect argument types without
const seem to have been taken from erroneous man pages.

fix sigorset/sigandset: _NSIG/8 is the size in bytes

sigandset/sigorset: do not check for NULL pointers.

that way it's consistent with existing sig* functions, and saves
some code size.

fixup sigandset

add sigandset and sigorset (needed for qemu)