* spawn.cc (spawn_guts): Revert patch which treated derived cygwin programs

* spawn.cc (spawn_guts): Revert patch which treated derived cygwin programs
differently from those which are mounted with -X.  Pass extra argument to
linebuf::fromargv.
* winf.h (MAXCYGWINCMDLEN): New define.
(linebuf::finish): Add a new argument denoting when command line overflow is
ok.
(linebuf::fromargv): Ditto.
* winf.cc (linebuf::finish): Implement above change.
(linebuf::fromargv): Ditto.