ktrace(1): add more xrefs

[emaste]

Summary

After a87651e2ff18 kib@ suggested cross-referencing intro(2) and sigaction(2).

[emaste]

Renders as:

The kernel operations that are traced include system calls (see intro(2)),
namei(9) translations, signal processing (sigaction(2)), and I/O.

[kib]

usr.bin/ktrace/ktrace.1

50–51

Might be convert namei reference in the similar way:

file system path lookups
.Pq Xr namei 9 ,

[emaste]

Update with @kib's suggestion.

[emaste]

The ktrace utility enables kernel trace logging for the specified
processes.  Kernel trace data is logged to the file ktrace.out.  The
kernel operations that are traced include system calls (see intro(2)),
file system path lookups (namei(9)), signal processing (sigaction(2)),
and I/O.

[emaste]

add a namei xref in the option section, suggested by @arrowd