Set bpmodify usage function
Set bpmodify usage function

golang `flag` package's default FlagSet `flag.CommandLine` calls
`flag.Usage` and `os.Exit(2)` on error to print the usage string.
Set `flag.Usage` to our custom usage function.

Test: m bpmodify; bpmodify -h; bpmodify --help
Change-Id: Ida107b0dbb07c291c3d7ea90eda9147d04a7cd51

danw/blueprint,google/blueprint,google/blueprint,danw/blueprint