Add manpage for rctl_* system calls

[badger]

Summary

I tried to avoid duplicating information in the rctl(8) manpage while still
making this manpage useful in its own right.

[trasz]

Looks great, thanks!

[wblock]

Very nice! My only suggestions are:

s/comma separated/comma-separated/

and

change the very formal "excepting differences described in" language to "with exceptions described in".

[badger]

Minor tweaks:

• s/comma separated/comma-separated/
• Shorten long function signature lines
• s/excepting differences noted in/with exceptions noted in/

[wblock]

Works for me. Please run mandoc -Tlint and igor -R on it to check for unexpected problems before commit.

Thanks!