kernel: Add the KERN_PROC_PATHNAME sysctl

Which returns the full path of a process text file.

Obtained-from: FreeBSD
A few accumulated manual page fixes.
rc.conf: Fix typo in a comment.
rc.conf: Set default for sysvipcd(8) and document.
<sys/sysctl.h>: Fix indent.
sysvipcd(8): It is -p which takes an argument, not -f.
sysvipcd(8): Make -d imply -f and write a usage().
sysvipcd(8): Use pidfile() and add missing header.
sysvipc.8: Add description for -p.
Adjust i386 for the sysvipc work.