This commit fixes rights check for `fd_pread` and `fd_pwrite` to be
conformant with the WASI spec. In the spec, it is clearly stated that
the right to invoke `__wasi_fd_pread()` requires a combination of
`__WASI_RIGHT_FD_READ` with `__WASI_RIGHT_FD_SEEK`, and similarly for
`__wasi_fd_pwrite()` the combination is `__WASI_RIGHT_FD_WRITE` with
`__WASI_RIGHT_FD_SEEK`. Relevant link to the spec: [__wasi_rights_t].
[__wasi_rights_t]: https://github.com/WebAssembly/WASI/blob/master/phases/snapshot/docs/wasi_unstable_preview1.md#__wasi_rights_t-uint64_t-bitfield
* Dynamically load utimensat if exists on the host
This commit introduces a change to file time management for *nix based
hosts in that it firstly tries to load `utimensat` symbol, and if it
doesn't exist, then falls back to `utimes` instead. This change is
borrowing very heavily from [filetime] crate, however, it introduces a
couple of helpers and methods specific to WASI use case (or more
generally, to a use case which requires modifying times of entities
specified by a pair `(DirFD, RelativePath)` rather than the typical
file time specification based only absolute path or raw file descriptor
as is the case with [filetime] crate. The trick here is, that on kernels
which do not have `utimensat` symbol, this implementation emulates this
behaviour by a combination of `openat` and `utimes`.
This commit also is meant to address #516.
[filetime]: https://github.com/alexcrichton/filetime
* Fix symlink NOFOLLOW flag setting
* Add docs and specify UTIME_NOW/OMIT on Linux
Previously, we relied on [libc] crate for `UTIME_NOW` and `UTIME_OMIT`
constants on Linux. However, following the convention assumed in
[filetime] crate, this is now changed to directly specified by us
in our crate.
[libc]: https://github.com/rust-lang/libc
[filetime]: https://github.com/alexcrichton/filetime
* Refactor UTIME_NOW/OMIT for BSD
* Address final discussion points
