Dynamically load utimensat if exists on the host (#535)

* 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

crates/wasi-common/src/sys/unix/mod.rs

@@ -3,6 +3,7 @@ pub(crate) mod host_impl;
 pub(crate) mod hostcalls_impl;
 
 mod dir;
+mod filetime;
 
 #[cfg(any(
     target_os = "macos",