Add support for a "[" that conforms to the GNU coding standards,

Add support for a "[" that conforms to the GNU coding standards,
i.e., that does not depend on its name.

(LBRACKET): Define to 0 if not defined.
(main): Use LBRACKET rather than argv[0].

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(one_argument): Do not check for -t if POSIXLY_CORRECT.

(main): Do not recognize --help or --version if
POSIXLY_CORRECT, when invoked as "test".  Handle "[ ]" correctly.
Do not bother testing that margv[margc] is non-null.