Add -m and -o flags to sort(1)

Sort comes pretty much automatically, as no script relies on the
undefined behaviour of the input _not_ being sorted, we might as well
sort the sorted input already.
The only downside is memory usage, which can be an issue for large
files.
The o-flag was trivial to implement.

README

@@ -66,7 +66,7 @@ The following tools are implemented ('*' == finished, '#' == UTF-8 support,
 =*| sha256sum       non-posix                    none
 =*| sha512sum       non-posix                    none
 =*| sleep           yes                          none
-    sort            no                           -m, -o, -d, -f, -i
+    sort            no                           -d, -f, -i
 =*| split           yes                          none
 =*| sponge          non-posix                    none
 #*| strings         yes                          none