add public declaration for optreset under appropriate feature profiles

add public declaration for optreset under appropriate feature profiles

commit 030e52639248ac8417a4934298caa78c21a228d1 added optreset, a BSD
extension to getopt duplicating the functionality (also an extension)
of setting optind to 0, but failed to provide a public declaration for
it. according to the BSD documentation and headers, the application is
not supposed to need to provide its own declaration.