The new 'genode_envp' variable declared in '_main.cc' allows libc
plugins to supplying custom environment pointers to the main function.
This is needed by 3rd-party software such as GNU make, which expects the
environment pointer as third argument of the main function.
This commit unifies the policy name for the template argument for
Native_capability_tpl to Cap_dst_policy, like suggested by Norman in the
discussion resulting from issue #145. Moreover, it takes the memcpy
operation for copying a Native_capability out of the template, which is
included by a significant bunch of files, and separates it in a library,
analog to the suggestion in issue #145.
