Expand and clarify libc/kernel sigset_t differences. Also move up the signature for rt_sigprocmask, similar to the way this is done in clone.2. Due to the history of sigprocmask, there are various notions of what sigset_t refers to. This attempts to clarify the man page, by giving the major instances different names: - sigset_t is the glibc sigset_t (1024 bits) - kernel_sigset_t is the kernel's sigset_t (64 bits) - old_kernel_sigset_t is the pre-rt kernel's sigset_t (32 bits) and explaining their difference in the NOTES. Even though the sources do not refer to the various sigset_t's by these names, I think it is important to be explicit, esp since sizeof(sigset_t) would give an incorrect value for `sigsetsize` if written in a source file that includes the libc headers. Lastly, move the note on an incorrect `sigsetsize` causing EINVAL up to the ERRORS section, so everything is in one place.