std: Fix a doc example on io::signal

This also makes the listener struct sendable again by explicitly putting the
Send bound on the relevant Rtio object.

cc #13352
auto merge of #13354 : alexcrichton/rust/fixup-some-signals, r=sfackler

This also makes the listener struct sendable again by explicitly putting the
Send bound on the relevant Rtio object.

cc #13352
Improve searching for XXX in tidy script (#3303)

Few places where previous version of tidy script cannot find XXX:
* inside one-line comment preceding by a few spaces;
* inside multiline comments (now it finds it if multiline comment starts
on the same line with XXX).
Change XXX to FIXME (closes #3303)

Change occurences of XXX found by new tidy script.