Make os::getcwd() return IoResult<Path>

os::getcwd() panics if the current directory is not available. According
to getcwd(3), there are three cases:

- EACCES: Permission denied.
- ENOENT: The current working directory has been removed.
- ERANGE: The buffer size is less than the actual absolute path.

This commit makes os::getcwd() return IoResult<Path>, not just Path,
preventing it from panicking.

As os::make_absolute() depends on os::getcwd(), it is also modified to
return IoResult<Path>.

Fixes #16946.

[breaking-change]

src/test/run-pass/process-spawn-with-unicode-params.rs

@@ -26,7 +26,7 @@ use std::path::Path;
 
 fn main() {
     let my_args = os::args();
-    let my_cwd  = os::getcwd();
+    let my_cwd  = os::getcwd().unwrap();
     let my_env  = os::env();
     let my_path = Path::new(os::self_exe_name().unwrap());
     let my_dir  = my_path.dir_path();

src/test/run-pass/tempfile.rs

@@ -123,7 +123,7 @@ fn test_rm_tempdir_close() {
 // to depend on std
 fn recursive_mkdir_rel() {
     let path = Path::new("frob");
-    let cwd = os::getcwd();
+    let cwd = os::getcwd().unwrap();
     println!("recursive_mkdir_rel: Making: {} in cwd {} [{}]", path.display(),
            cwd.display(), path.exists());
     fs::mkdir_recursive(&path, io::USER_RWX);
@@ -141,7 +141,7 @@ fn recursive_mkdir_dot() {
 
 fn recursive_mkdir_rel_2() {
     let path = Path::new("./frob/baz");
-    let cwd = os::getcwd();
+    let cwd = os::getcwd().unwrap();
     println!("recursive_mkdir_rel_2: Making: {} in cwd {} [{}]", path.display(),
            cwd.display(), path.exists());
     fs::mkdir_recursive(&path, io::USER_RWX);