std: Changing the meaning of the count to splitn

This commit is an implementation of [RFC 979][rfc] which changes the meaning of
the count parameter to the `splitn` function on strings and slices. The
parameter now means the number of items that are returned from the iterator, not
the number of splits that are made.

[rfc]: https://github.com/rust-lang/rfcs/pull/979

Closes #23911
[breaking-change]

src/librustdoc/lib.rs

@@ -333,7 +333,7 @@ fn acquire_input(input: &str,
 fn parse_externs(matches: &getopts::Matches) -> Result<core::Externs, String> {
     let mut externs = HashMap::new();
     for arg in &matches.opt_strs("extern") {
-        let mut parts = arg.splitn(1, '=');
+        let mut parts = arg.splitn(2, '=');
         let name = match parts.next() {
             Some(s) => s,
             None => {