Add cap_iab_{compare,get_pid} functions to libcap; --iab to getpcaps.

This brings libcap back to parity with the Go 'cap' package. We
provide a CAP_IAB_DIFFERS(result, vector) macro to evaluate the result
of cap_iab_compare().

Extend the getpcaps arguments to include --iab. This causes the utility
to explore the IAB tuple for the specified process. When used, this
outputs a text representation in a similar format to that of the
'captree' (Go) utility.

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>