exec: factor out most function arguments of exec_spawn() to ExecParameters

exec: factor out most function arguments of exec_spawn() to ExecParameters

This way, the list of arguments to that function gets more comprehensive,
and we can get around passing lots of NULL and 0 arguments from socket.c,
swap.c and mount.c.

It also allows for splitting up the code in exec_spawn().

While at it, make ExecContext const in execute.c.