Trivial patch to put the roff(7) \g (interpolate format of register)

Trivial patch to put the roff(7) \g (interpolate format of register)
escape sequence into the correct parsing class, ESCAPE_EXPAND.
Expansion of \g is supposed to work exactly like the expansion
of the related escape sequence \n (interpolate register value),
but since we ignore the .af (assign output format) request,
we just interpolate an empty string to replace the \g sequence.

Surprising as it may seem, this actually makes a formatting difference
for deviate input like ".O\gNx" which used to raise bogus "escaped
character not allowed in a name" and "skipping unknown macro" errors
and printed nothing, whereas now it correctly prints "OpenBSD".