fuse(4): convert debug printfs into dtrace probes

fuse(4) was heavily instrumented with debug printf statements that could
only be enabled with compile-time flags. They fell into three basic groups:

1. Totally redundant with dtrace FBT probes. These I deleted.
2. Print textual information, usually error messages. These I converted to SDT probes of the form fuse:fuse:FILE:trace. They work just like the old printf statements except they can be enabled at runtime with dtrace. They can be filtered by FILE and/or by priority.
3. More complicated probes that print detailed information. These I converted into ad-hoc SDT probes.

Sponsored by: The FreeBSD Foundation