Fix compiler warnings in iostat

Raise WARNS from 1 to 6 (the default)
Fix warnings:

• Use C99 designated initializers for structs, and initialize all fields
• Mark global variables as static
• Mark unused function arguments
• Be careful about signed/unsigned comparisons

Reviewed by: eadler
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D5328