Memory-usage data on platforms without /proc

[dmacks]

OS X doesn't have the /proc pseudo-filesystem, so running GDSIIConvert logs:

12/15/19::12:38:01: Mem[0] before,during,after={0,0,0},delta=0
12/15/19::12:38:01: Mem[1] before,during,after={0,0,0},delta=0
12/15/19::12:38:01: Mem[2] before,during,after={0,0,0},delta=0
12/15/19::12:38:01: Mem[3] before,during,after={0,0,0},delta=0
12/15/19::12:38:01: Mem[4] before,during,after={0,0,0},delta=0
12/15/19::12:38:01: Mem[5] before,during,after={0,0,0},delta=0
12/15/19::12:38:01: Mem[6] before,during,after={0,0,0},delta=0

I assume Windows has a similar result. It would be better to log that the data wasn't available (absence of data != actual data value of zero). GetMemoryUsage() is prototyped to return an unsigned int, but hardcoded to return 0 and that value is ignored. If it returned a value that signifies success vs failure, then the caller (main()) can call ::Log with a different string to indicate this situation.