Add initial support for the ambient set.

The ambient set is some strangeness associated with trying to revive
naive inheritance. While personally not a fan of this feature, I
recognize it is in the kernel so libcap now supports it with
three new functions:

  int cap_get_ambient(cap_value_t cap)
  int cap_set_ambient(cap_value_t cap, cap_flag_value_t set)
  int cap_reset_ambient(void)

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