Stop calling system calls "function calls".
Use "The .Fn system call" a-la "The .Nm utility".
When referring to a non-BSD implementation in
the HISTORY section, call syscall a function,
to be safe.
Previously, some useful xrefs were missing.
Now each of the pages refers to all remaining section 2 pages,
to the kld(4) page, and to a related utility's (section 8) page.
