kdump: decode pollfd struct arrays coming from poll(2)

We'll handle these just as we do kevents, one per line with subsequent
lines indented sufficiently to distinguish them from the upcoming
return value.

Sample, with indentation stripped and revents changed to '...' in the
first one to keep the line length down:

CALL poll(0x820610560,0x3,0)
STRU struct pollfd[] = { { fd=0, events=0x1<POLLIN>, revents=0x11<...>
{ fd=1, events=0x4<POLLOUT>, revents=0x4<POLLOUT>}
{ fd=-1, events=0x4<POLLOUT>, revents=0} }
RET poll 2

Reviewed by: bapt, jhb
Differential Revision: https://reviews.freebsd.org/D44160