Make abidiff and abicompat return meaningful exit codes

Make abidiff and abicompat return meaningful exit codes

As per https://sourceware.org/bugzilla/show_bug.cgi?id=18146, abidiff
the exit code of abidiff and abicompat is now a bit field that can be
inspected to know if the ABI change reported is incompatible for sure,
or if it needs user review of the output to decide.

This patch also updates the documentation.

	* doc/manuals/abicompat.rst: Update documentation for abicompat
	exit codes.
	* doc/manuals/abidiff.rst: Likewise for abidiff exit codes.
	* include/abg-tools-utils.h (enum abidiff_status): Declare new
	enum.
	(operator{|,&,|=}): Declare new operators for the new enum
	abidiff_status.
	(abidiff_status_has_error, abidiff_status_has_abi_change)
	(abidiff_status_has_incompatible_abi_change): Declare new
	functions.
	* src/abg-tools-utils.cc (operator{|,&,|=}): Define these new
	operators.
	(abidiff_status_has_error, abidiff_status_has_abi_change)
	(abidiff_status_has_incompatible_abi_change): Define new
	functions.
	* tests/test-diff-filter.cc (main): Adjust for the new exit code
	of abidiff.
	* tests/test-diff-suppr.cc (main): Likewise.
	* tests/test-abicompat.cc (main): Likewise.
	* tools/abicompat.cc (enum abicompat_status): Remove.
	(operator{|,&,|=}): Remove these operators for enum
	abicompat_status.
	(perform_compat_check_in_normal_mode)
	(perform_compat_check_in_weak_mode): Return abidiff_status instead
	of abicompat_status.  Adjust therefore.
	(main): Adjust to return abidiff_status now, instead of a just
	zero for all non-error cases.
	* tools/abidiff.cc (main): Likewise.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>