I would like to print target_waitkind values in debug messages, so I
think that a target_waitkind-to-string function would be useful. While
at it, use it in target_waitstatus::to_string. This changes the output
of target_waitstatus::to_string a bit, but I think it is for the better.
The debug messages will show a string matching exactly the
target_waitkind enumerator (minus the TARGET_WAITKIND prefix).
As a convenience, make string_appendf return the same reference to
string it got as a parameter. This allows doing this:
return string_appendf (str, "foo");
... keeping the code concise.
Change-Id: I383dffc9c78614e7d0668b1516073905e798eef7
