-S flag fixes for DOSISH platforms

-S flag fixes for DOSISH platforms

This patch supercedes the one posted here by Ilya
(Message-Id: <199707191651.MAA04897@monk.mps.ohio-state.edu>).

There are no changes for Unix platforms over Ilya's
version.  On DOSISH platforms, the initial check in
the current directory (or the actual path to the script,
if supplied) includes searching for valid extensions.

The fact that -S does not do a PATH search if the supplied
filename contains directory separators (on all platforms) is
documented.  This behavior is similar to Unix and DOS shells.

Note -S *does* have an effect on DOSISH platforms even if no
PATH search happens: valid extensions will be checked for if
the file name is not found.

p5p-msgid: 199707250043.UAA02385@aatma.engin.umich.edu