Add a trait for non-path C string?

[SteveLauC]

I am thinking about adding another trait to represent string, the definition will be same as our NixPath trait, and these 2 traits only differ in semantics, NixPath is for path, this new trait is for string.

By adding this trait, we split "C string" (C does not have a string type, it is only a sequence of chars followed by a NUL) into 2 categories by semantics:

1. path
2. string

Currently, it will be used in 2 interfaces if added:

1. The ident argument of openlog() on non-Linux systems

   nix/src/syslog.rs

   Lines 45 to 47 in 0c40b8d

   	#[cfg(not(target_os = "linux"))]
   	pub fn openlog<S: AsRef<OsStr> + ?Sized>(
   	ident: Option<&S>,

   From man page:

   The string pointed to by ident is prepended to every message, and is typically set to the program name.

2. The path argument of posix_spawnp()

   nix/src/spawn.rs

   Lines 396 to 397 in 0c40b8d

   	pub fn posix_spawnp<SA: AsRef<CStr>, SE: AsRef<CStr>>(
   	path: &CStr,

   It is the name of the program that the spawned child process will execute

   This argument should not be called path, I will rename it to file

---

Alternative approaches

1. Just use the NixPath trait

   It will definitely work, though it will feel weird semantically due to the Path in its name. Or maybe we can rename the trait to something like NixCString?

2. Use S: AsRef<OsStr>

   openlog() currently uses this approach, after converting it to &OsStr we can use NixPath under the hood, so it is convenient:

   <OsStr as NixPath>::with_nix_path(os_str.as_ref(), |c_str| {
       // do something with c_str
   });

   The only flaw of this method IMHO is that users cannot use &CStr because CStr is not AsRef<OsStr>, but &CStr should be allowed.

[SteveLauC]

cc @asomers, thoughts on this?