Historically guc.c has just refused examples like set work_mem = '30.1GB',
but it seems more useful for it to take that and round off the value to
some reasonable approximation of what the user said.  Just rounding to
the parameter's native unit would work, but it would lead to rather
silly-looking settings, such as 31562138kB for this example.  Instead
let's round to the nearest multiple of the next smaller unit (if any),
producing 30822MB.

Also, do the units conversion math in floating point and round to integer
(if needed) only at the end.  This produces saner results for inputs that
aren't exact multiples of the parameter's native unit, and removes another
difference in the behavior for integer vs. float parameters.

In passing, document the ability to use hex or octal input where it
ought to be documented.

Discussion: https://postgr.es/m/1798.1552165479@sss.pgh.pa.us