rename vfs.zfs.debug_flags to vfs.zfs.debugflags

While the former name is easier to read, the "_flags" suffix has a special
meaning for loader(8) and, thus, it was impossible to set the knob via
loader.conf(5). The loader interpreted the setting as flags that should
be passed to a kernel module named "vfs.zfs.debug".

Discussed with: smh
MFC after: 2 weeks