Where available, use sysctl() with KERN_PROC_PATHNAME to make $^X absolute.

Where available, use sysctl() with KERN_PROC_PATHNAME to make $^X absolute.

In Configure, check whether sysctl() and KERN_PROC_PATHNAME can be used
to find the absolute pathname of the executable. If so, set
usekernprocpathname in config.sh and USE_KERN_PROC_PATHNAME in config.h.
If this is set, then use this approach in S_set_caret_X() to canonicalise
$^X as an absolute path. This approach works on (at least) FreeBSD, and
doesn't rely on the /proc filesystem existing, or /proc/curproc/file being
present.