gdb: add mailing list and IRC information to --help

A few user-vibisble changes to the --help output:

 * Remove unnecessary quotes around bug url.
 * Mention the mailing list and IRC channel as places where users can
   ask GDB-related questions.
 * Add empty lines between items in the footer, to improve readability.
 * Remove unnecessary new line at the end of output.

2020-06-09  Jonny Grant  <jg@jguk.org>
2020-06-09  Simon Marchi  <simon.marchi@polymtl.ca>

	* main.c (captured_main_1): Don't print new line after help.
	(print_gdb_help): add mailing list and IRC channel information
	to --help.  Add new lines between items in the footer.  Remove
	quotes around bug url.

Signed-off-by: Jonathan Grant <jg@jguk.org>
Change-Id: Ibd0746a348d558fb35b5cd7e366f107742806565