bgpd: Fix group overrides for inverted AF flags

bgpd: Fix group overrides for inverted AF flags

This commit fixes peer-group overrides for inverted AF flags. This
implementation is currently only being used by the three 'send-community'
flags. Commit 70ee29b4d introduced generic support for overriding AF
flags, but did not support inverted flags.

By introducing an additional array on the BGP peer structure called
'af_flags_invert' all current and future flags which should work in an
inverted way can now also be properly overridden.

The CLI commands will work exactly the same way as before, just that 'no
<command>' now sets the flag and override whereas '<command>' will unset
the flag and remove the override.

Signed-off-by: Pascal Mathis <mail@pascalmathis.com>