syscallsubr.h: Header inclusions: Explicit <sys/types.h>, sort

[olce]

Summary

<sys/types.h> is currently brought in by other headers but some of its
type definition are directly used in this header, so it should appear
explicitly. It is necessary as lots of prototypes in there use types it
defines (<sys/_types.h> wouldn't be enough).

Sort header inclusions as per style(9).

No functional change (intended).

[imp]

So if sys/_types.h won't suffice, I'd call out why in the commit message (this file makes no pretense of being namespace clean I think would be the reason.)

[olce]

In D47881#1091936, @imp wrote:

So if sys/_types.h won't suffice, I'd call out why in the commit message (this file makes no pretense of being namespace clean I think would be the reason.)

Commit message amended.