remove/restore effective capabilities

remove/restore effective capabilities

This patch adds the functions to drop the 'effective' capabilities and
restore them from the 'permitted' capabilities.

When the command is run as 'root' we do nothing.
When the command is run as 'lambda' user, we drop the effective capabilities
When the command is run as 'root' but real uid is not root, we keep the capabilies,
switch to real uid, and drop the effective capabilities.

This approach is compatible for root user, lambda + file capabilities
and lambda + setuid.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>