Introduce two helpers, the more general SYSCTL_SIZEOF() and
a struct-specific one SYSCTL_SIZEOF_STRUCT() which prepends 'struct' in
the description and in the use of sizeof() but uses the raw structure
name as the knob's name. The size of the object/structure is exported
under 'debug.sizeof'.
Existing knobs under 'debug.sizeof' were all converted to use the helpers.
Add a note before the helpers that no new sysctl knobs should be added
under 'debug.sizeof' and list alternative, generic means to obtain the
size of arbitrary kernel structures (thanks to markj@ for providing these).
No functional change (intended).