Add support for multi-threading of soft updates.

Replace a single soft updates thread with a thread per FFS-filesystem
mount point. The threads are associated with the bufdaemon process.

Reviewed by:  kib
Tested by:    Peter Holm and Scott Long
MFC after:    2 weeks
Sponsored by: Netflix
Integrate lib/libmp into the build/kyua

- Remove the .t wrapper
- Fix -Wreturn-type warnings with clang

This change has been tested on amd64/i386

Phabric: D530
Reviewed by: jmmv
Approved by: jmmv (co-mentor)
MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division