install: Fix argument parsing for '--backup'

The '--backup' option would previously accept arguments separated from the
option either by a space or an equals sign. The GNU implementation strictly
requires an "equals" for argument separation.

As the argument to '--backup' is optional, the equals sign mustn't be ommited
as otherwise there is no way to tell a file argument apart from an argument
that's meant for the '--backup' option. This ensures that if '--backup' is
present it either has no further associated arguments (i.e. fallback to the
default), or the arguments are separated by an equals sign.

src/uu/install/src/install.rs

@@ -106,6 +106,9 @@ pub fn uumain(args: impl uucore::Args) -> i32 {
                 Arg::with_name(OPT_BACKUP)
                 .long(OPT_BACKUP)
                 .help("(unimplemented) make a backup of each existing destination file")
+                .takes_value(true)
+                .require_equals(true)
+                .min_values(0)
                 .value_name("CONTROL")
         )
         .arg(