sigaltstack(2): document some modernizations: * un-document 'struct sigaltstack' tag for stack_t as this is BSD-specific;   this doesn't seem useful enough to document as such * alternate stacks are per thread, not per process * update error codes to what the kernel does and POSIX requires
sigaltstack(2): document some modernizations:
* un-document 'struct sigaltstack' tag for stack_t as this is BSD-specific;
  this doesn't seem useful enough to document as such
* alternate stacks are per thread, not per process
* update error codes to what the kernel does and POSIX requires

MFC after:	1 week

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase