CHERI-style compact printing format

CHERI-style compact printing format

This patch adds CHERI-style compact printing format to GDB and uses it
by default.

The format is described here: https://github.com/CTSRD-CHERI/cheri-c-programming/wiki/Displaying-Capabilities

gdbsupport/ChangeLog:

2020-12-16  Luis Machado  <luis.machado@arm.com>

	* capability.cc (cap_short_perms_strings): Remove.
	(capability::is_null_derived): New member function.
	(capability::to_str): Document and handle new CHERY-style printing
	format.
	* capability.h (cap_constants): Document the CAP_SEAL_TYPE_*
	constants.
	(struct capability) <is_null_derived>: New member function prototype.