Updates and enhancements to io.d to aid DTrace scripting

Updates and enhancements to io.d to aid DTrace scripting

+ Add dev_type do devinfo_t
+ Add b_cmd to bufinfo_t
+ Add constants for BIO_* and DEVSTAT_TYPE_*
+ Add inline for converting BIO_* int to string
+ Add inline for converting DEVSTAT_TYPE_* int to string
+ Add mask for dev_type & DEVSTAT_TYPE_MASK to string
+ Add mask for dev_type & DEVSTAT_TYPE_IF_MASK to string

Reviewed by:	markj
Sponsored by:	Smule, Inc.
Differential Revision:	https://reviews.freebsd.org/D14396