(seq invocation): seq now uses long double

(seq invocation): seq now uses long double
internally rather than double.  It now defaults to a minimal fixed
point format if possible.  It lets you use %a, %A, %E, %F, %G.
Don't assume printf doesn't work for numbers that fit in 64 but
not 32 bits; typically they work these days.  Improve discussion
of large integers and update the rounding-error numbers.