Changes to WASI spec in snapshot_preview2

[cjihrig]

Changes that need to be addressed in uvwasi

Work is going on in the snapshot2 branch. Progress is indicated in the list below.

• The __wasi_proc_raise() function and the __wasi_signal_t type and constants were removed in Remove proc_raise and the signal types and constants. WebAssembly/WASI#136. This change impacts ephemeral only.
• The functions __wasi_path_permissions_set() and __wasi_fd_permissions_set() were added in Add functions for working with file permissions. WebAssembly/WASI#170.
• The __wasi_poll_oneoff() function should return EINVAL if nsubscriptions is zero. Make poll return errno::inval if there are no subscriptions. WebAssembly/WASI#193.
• The uvwasi_filetype_t added UVWASI_FILETYPE_FIFO in Add FIFO filetype to match POSIX WebAssembly/WASI#189.
• The UVWASI_O_CREAT flag should be renamed to UVWASI_O_CREATE, in accordance with Rename creat to create. WebAssembly/WASI#199.
• Union formats have changed in Change witx unions to be tagged WebAssembly/WASI#220. See Update to use witx tagged unions WebAssembly/wasi-libc#165.
• The clocks process_cputime_id and thread_cputime_id were removed in Remove the cputime clocks. WebAssembly/WASI#197.
• The error acces was renamed to access in Rename acces to access #209 WebAssembly/WASI#211.
• The uvwasi_dirent_t fields have been rearranged to move the d_namlen field to the end of the struct. See Move $d_namlen to the end of $dirent WebAssembly/WASI#264.