Hi everyone.

I noticed that `<unistd.h>` doesn't define `sync`, `crypt`, and some other
functions. Looked into the man page `unistd.h.0p` and sees nothing about
XSI in the function declarations.

Looked at the website, you can see the clear XSI refrences here^[1].

Needed to define `_XOPEN_SOURCE` to make them work.

This information should be avaliable to the man-page reader.
But it was not included by the automatic manual generation.

The XSI requirement information was in the format (raw HTML):

<sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup>
<img src="../images/opt-start.gif" alt="[Option Start]" border="0">
...functions that needs XSI extenion support goes here...
<img src="../images/opt-end.gif" alt="[Option End]" border="0">

Note that this can include more than one function at a time.

The guys at IEEE suck designing anything.

- [1] https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/unistd.h.html#tag_13_77_03_06