Fix check-plist of USES=kmod

No longer depend on (KERN_)DEBUGDIR being non-empty to decide whether to
create the empty KERN_DEBUGDIR/KMODDIR directory, instead use a new
argument 'debug' to indicate this.

Add KERN_DEBUGDIR to PLIST_SUB and MAKE_ENV so that the including ports
can use them.

Ports using kmod:debug do need to add @dir entries to their pkg-plist for
now to prevent 'make check-plist' errors, for example for default KMODDDIR:

@dir /%%KERN_DEBUGDIR%%/%%KMODDIR%%
@dir /%%KERN_DEBUGDIR%%/boot
@dir /%%KERN_DEBUGDIR%%

It would indeed be nice to automate this.

PR: 216879
Submitted by: Sergey Kozlov (kozlov.sergey.404@gmail.com)