Add support to passing directories to EXTRA_PATCHES.

Instead of:

  EXTRA_PATCHES=	${PATCHDIR}/feat-foo-patch-file.c \
			${PATCHDIR}/feat-foo-patch-file.h \
			${PATCHDIR}/feat-foo-patch-file2.c

Move those files to ${PATCHDIR}/feat-foo/, name them patch-*, and set:

EXTRA_PATCHES=	${PATCHDIR}/feat-foo

Approved by: bdrewery
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D10855