* Calculate growth size in an initial loop.
* Add equivalent to str_ensure_available_capa for growing strings.
* Refactor for arity-split (just 0 and 1 arg forms added).

If a thread is killed and then quickly interrupted in another way,
it may end up polling for that other event and stop aborting from
the kill. This ensures that once aborted a thread can never stop
aborting by picking up a new interrupt event.

This broke the new test server in test/mri/net/http/utils.rb when
it killed the server and then closed the socket; while aborting
the closed socket could trigger additional exceptions, stopping
the abort. This ensures it will be seen at the next poll and
resumed.

See ruby/ruby#5237

In CRuby, a thread remains internally in "run" state while in the
process of being killed, but a killed bit is set for other
status-sensitive functions to check. This patch aligns behavior
with CRuby.

This is a temporary move until we can follow CRuby and implement
a native version of this function.

See jruby#8476

"aborting" is only presented as status when a thread has been
actively killed, which is implemented by a separate flag. The
thread remains internally in RUN status, as in CRuby.

Copied from my PR to fix this double-interrupt race.

See ruby/net-http#197