check.cmd: fix that check fails due to an empty header list

'until' is evaluated at the end of loop not at the entrance.

    modified:   check.cmd
Implement spawnvpe() to support a very long command line

    modified:   check.cmd
    new file:   process/spawnvpe.c
    new file:   testcase/spawnvpe-1-child.c
    new file:   testcase/spawnvpe-1.c
Introduce a build system for testcases

    new file:   testcase/Makefile
    new file:   testcase/Makefile.common
Add .gitattributes

REXX expects a rexx file to have CR/LF eol style.

    new file:   .gitattributes