Use safe conversion functions for parsing RTS number arguments.

Motivation

The GHC RTS currently gladly accepts bogus input like ghc +RTS -ibogus.

The reason for this is that the rest of the string after -i will be passed to atof. atof will fail to convert it to a number and silently returns zero.

I've recently spent far too long trying to debug an error where it turned out I used - instead of --. Handling this as proposed below would have made this obvious immediately.

Proposal

GHC Should report a error in cases like the example above. This can likely be solved by using strtof instead of atof and checking for failed conversions.