[PATCH] Complete example in error message documentation.

[ricaljasan]

The manual gives "an example showing how to handle failure to open a
file correctly."  The example function, open_sesame, uses the
newly-introduced strerror function and errno and
program_invocation_short_name variables.  It fails to specify GNU
extensions, however, so attempts to use it in the following way:

    int main (void) {open_sesame ("badname");}

fail during compilation with "error: ‘program_invocation_short_name’
undeclared", indicating the example is incomplete.  The presence of
for the function to work should be present.  For completeness, the
example is lacking the following line:

    #define _GNU_SOURCE

as the declarations of program_invocation_*name in errno.h are wrapped
in an "#ifdef __USE_GNU" conditional.

The documentation of the variables is also expanded, adding that their
definition lies in errno.h and noting specifically they are GNU
extensions.

    manual/errno.texi: Complete example function and expand
    documentation of program_invocation*_name variables.

2014-10-25  Rical Jasan  <redacted>

	* manual/errno.texi (Error Messages): Complete example function
	by adding missing #define.
	(program_invocation_name): Add statement indicating GNU
	extension and reference which header file declares the variable.
	(program_invocation_short_name): Likewise.

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