JNI/JSSE: Add support for poll(), fix for EAGAIN and select()

[cconlon]

Addition of poll() Support:

This PR adds support for using poll() by default instead of select() on native file descriptors. The previous select() behavior was left in tact and used as a default for Windows builds still. select() can also be used by building the native JNI library with WOLFJNI_USE_IO_SELECT defined. See java.sh cflags section for where that could be added.

We do not monitor lots of file descriptors at once ourselves inside wolfSSL JNI/JSSE, but one disadvantage to using select() is that for file descriptor numbers larger than FD_SETSIZE, the FD_ISSET and related macros result in undefined behavior. FD_SETSIZE defaults to 1024 on most systems, so is not very scalable for consumers using wolfSSL JNI/JSSE in processes that are opening and using lots of file descriptors.

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Changes to select() Support:

Native JNI socketSelect() loops around calling select() if it returns -1 and errno == EINTR. According to the select(2) man page, some variants of Unix may return EAGAIN from select(). This PR adjusts our socketSelect() to also loop on EAGAIN.

VERSIONS
       On some other UNIX systems, select() can fail with the error
       EAGAIN if the system fails to allocate kernel-internal resources,
       rather than ENOMEM as Linux does.  POSIX specifies this error for
       poll(2), but not for select().  Portable programs may wish to
       check for EAGAIN and loop, just as with EINTR.

Man page for select() from OSX also shows a potential return of EAGAIN from select():

ERRORS
     An error return from select() indicates:

     [EAGAIN]           The kernel was (perhaps temporarily) unable to allocate the requested number of file descriptors.

Related to ZD 17962 (and 17385) - customer confirmed fix seems to be working well so far.