Surfaced when running sunfishcode/misc-tests. When trying to
truncate the file without the __WASI_RIGHT_PATH_FILESTAT_SET_SIZE
right, error __WASI_ENOTCAPABLE was correctly returned, however,
the guest fd pointer was not encoded to -1 in that case. This
commit fixes it by taking out the guest fd encoding out of the
conditional branch which turns out obsolete.
This adds the C WASI implementation as a new crate, wasmtime-wasi-c,
and adds a command-line flag to the wasmtime command-line driver to
select which WASI implementation to use.
