show_help: static string improvements

[jsquyres]

Fix warnings from older C compilers that do not like literal strings that are over 4095 characters long. Admittedly do this by hueristic: make each line in a show_help INI help file be its own string (rather than joining all the lines in a given [section] together). I.e., assume that each individual line is less than 4095 characters.

Also make a few other improvements:

• Remove an unused string in show_help.c
• Fixed a problem where leading spaces were trimmed from the help text files in the resulting C strings.
• Fixed a problem where blank lines were snipped from the help text files in the resulting C strings.
• Simplify the Python generation script by statically initialzing one large struct named help_files (vs. initializing a struct per help file, and then initializing a 2nd struct to map the filenames to all the individual per-file structs).
  • Also use C99-style name-specified initializers, making the resulting code mode human-readable.
  • Also add comments at the end of each INI section and filename to make the resulting code more human readable.
  • Use C99-style "{0}" to mark empty / end-of-array sentinel values.
• At run time, the individual lines of a given show_help section are opal_argv_join()'ed together and emitted. The alloc'ed string is cached (just in case it is used again -- which it likely won't be) and freed during opal_finalize().

The resulting generated static initialization code looks like this:

static file_entry help_files[] = {
    {
	.filename = "help-opal-wrapper.txt",
	.entries = (ini_entry[]) {
	    {
		.section = "no-language-support",
		.content = (const char*[]) {
		    "Unfortunately, this installation of Open MPI was not compiled with",
		    "%s support.  As such, the %s compiler is non-functional.",
		    NULL
		},
	    }, // End of section no-language-support
	    {0}
	}, // End of file help-opal-wrapper.txt
    },
    {0}
};

Fixes #13167