Refactor sepolicy.rules resolve

We resolve available partitions for sepolicy.rules when patching
boot and bind mount the partition by magiskinit.

For older devices, the previous logic won't work because the part name
is never readable.

Co-authored-by: topjohnwu <topjohnwu@gmail.com>

native/src/base/files.rs

@@ -1,5 +1,9 @@
 use std::ffi::CStr;
+use std::fs::File;
+use std::io;
+use std::io::BufRead;
 use std::os::unix::io::{AsRawFd, FromRawFd, OwnedFd, RawFd};
+use std::path::Path;
 
 use libc::{c_char, c_uint, mode_t, EEXIST, ENOENT, O_CLOEXEC, O_PATH};
 
@@ -131,3 +135,8 @@ pub extern "C" fn mkdirs(path: *const c_char, mode: mode_t) -> i32 {
         0
     }
 }
+
+pub fn read_lines<P: AsRef<Path>>(path: P) -> io::Result<io::Lines<io::BufReader<File>>> {
+    let file = File::open(path)?;
+    Ok(io::BufReader::new(file).lines())
+}