20201231.092811.cbecfb1

replace naughty fgetpos/poke/fsetpos with fseeko

fpos_t is intended to be an opaque type so reaching inside it
is not right.

Switch to fseeko/ftello which are the off_t equivalents of
fseek/ftell.

closes: https://github.com/wez/atomicparsley/issues/13