As well as substantial updates to the man pages, including
an explanation of how capabilities can be robustly used with
pthreads, implement a set of convenience functions in libcap
to make dropping privilege in careful ways straightforward.

These include adding the abstraction of libcap recommended
"modes" and cap_setuid() and cap_setgroups() functions.

The progs/quicktest.sh script and capsh has been extended
to validate each of these new functions.

Add convenience functions to the libcap/cap Go package.
Specifically, added Compare(), Differs(), GetSecbits() &
Secbits.Set(), GetMode() & Mode.Set(), SetUID() and
SetGroups().

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