warning.
* path.h (path_conv::set_cygexec): New function.
* spawn.cc (av::iscygwin): Eliminate.
(av::av): Don't initialize iscygwin.
(spawn_guts): Just use real_path.iscygexec for all tests.
(av::fixup): Short circuit test if .exe extension and known cygexec. Set
cygexec flag appropriately if we find that program uses cygwin1.dll.
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