manual: complete example in error message documentation

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
"#include"s suggest everything neccesary 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.

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