Use normal human units, not "correct" SI units, when parsing MB/GB etc

Summary:
Technically, "KB" is "kilobyte" which is 1000 bytes, and "KiB" is "kibibyte" which is 1024 bytes. I implemented this function "correctly", but in practice we mostly use it to parse things from `php.ini`, `mysql.cnf`, etc., which all use units of 1024 intead of 1000. This ends up being really confusing because you write `128M` in a file (technicall meaning "128 mebibytes") and then a setup warning says `137M` (tehcnically meaning "137 megabytes").

Use the standard normal units that everyone and everything uses instead of the "correct" units. If we have some cause for the "real" SI units later we can mess with this stuff when we hit that.

Test Plan: Saw the setup check in D10630 actually report what I wrote in the file instead of a weird SI value.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10631