sysutils/e2fsprogs: 1/2 rename to -core, strip FLAVORS and /sbin

This is the first commit that renames e2fsprogs to e2fsprogs-core,
makes the necessary changes to Makefiles, strips the FLAVORs that
determined how to link or copy to /sbin. The latter feature will
be placed into a new e2fsprogs port that just copies two files to /sbin.

A followup 2/2 commit will then add the new e2fsprogs port.

This is done so that Git can properly track the rename.

While here, switch from FreeBSD's dd to GNU coreutils's dd
in an attempt to fix sporadic m_offset self-test failures.