small cleanup to fsx.c

[kirussel]

I have proposed some changes that fixes the following:

• for msync(), uses MS_SYNC, instead of hard coding zero
• for mmap(), uses MAP_FAILED, instead of harding coding -1
• for getopt(), using -1, instead of EOF
• added "y" to the getopt string -- this is already implemented but is missing from the command line args

References

• https://fstools.macosforge.org/trac/ticket/14
• master...kirussel/misc