feat: add group configuration (+)

will only check users of such group

also cleaned some code:
- Input doesn't need to be UTF-8 valid.
- Main fuction is dead-simple, very readable.
- Added some modularity.
- Add some docs on behavior details.

src/ext.rs

@@ -1,4 +1,7 @@
-use std::ffi::CStr;
+use std::{
+    ffi::{CStr, OsStr},
+    os::unix::ffi::OsStrExt as _,
+};
 
 pub trait SliceExt {
     fn split_once_ch(&self, ch: u8) -> Option<(&[u8], &[u8])>;
@@ -13,10 +16,15 @@ impl SliceExt for [u8] {
 }
 
 pub trait CStrExt {
+    fn to_os_str(&self) -> &OsStr;
     fn split_ch(&self, ch: u8) -> Option<(&[u8], &CStr)>;
 }
 
 impl CStrExt for CStr {
+    fn to_os_str(&self) -> &OsStr {
+        OsStr::from_bytes(self.to_bytes())
+    }
+
     fn split_ch(&self, ch: u8) -> Option<(&[u8], &CStr)> {
         let self_slice = self.to_bytes_with_nul();
 
@@ -27,3 +35,13 @@ impl CStrExt for CStr {
         })
     }
 }
+
+pub trait StrExt {
+    fn as_os_str(&self) -> &OsStr;
+}
+
+impl StrExt for str {
+    fn as_os_str(&self) -> &OsStr {
+        OsStr::from_bytes(self.as_bytes())
+    }
+}