Add kld to CATEGORIES.

Set SSP_UNSAFE.

Set IGNORE if the kernel sources are not found in SRC_BASE.

Define KMODDIR to /boot/modules by default, add it to PLIST_SUB and MAKE_ENV, and create it upon installation. If KMODDIR is set to /boot/kernel, it will be rewritten to /boot/modules. This prevents breaking packages when upgrading the kernel due to /boot/kernel being renamed to /boot/kernel.old in the process.

Handle cross-referencing kernel modules upon installation and deinstallation, using @kld.