PerlIO: don't set the error flag when reading on an EAGAIN

[tonycoz]

This allows questionable code that tries to combine select and buffered I/O to limp along. Such code is still
risky due to select() checking the underlying OS handle and not the perl handle.

Fixes #22883 but is perhaps a bit more daring.

---

• This set of changes requires a perldelta entry, and I need help writing it.

[jkeenan]

This code in the pull request ...

+{
+    use IO::Select;
+

... causes a failure in make minitest.

$ ./miniperl -Ilib t/io/perlio.t
Can't locate IO/Select.pm in @INC (you may need to install the IO::Select module) (@INC entries checked: ... 
) at t/io/perlio.t line 250.
BEGIN failed--compilation aborted at t/io/perlio.t line 250.

Also failing on Windows runs in our CI.

[tonycoz]

Fixed minitest.

The Win32 failure was due to a missing SKIP: label (I adapted this from the other readline fix branch which used skip_alls) and that test now passes (and has passed in CI at this point)

[tonycoz]

I think an adaption of this might be useful for a longer term fix, but I don't think it's suitable this late in the release cycle, it changes the behaviour of read() (and getc() which is implemented in terms of get() read()) to effectively not return a failure condition which existing code may rely on.