ustfork: Initialize libc pointers in constructor

ustfork: Initialize libc pointers in constructor

Instead of resolving individual libc functions lazily at their call
site, resolve every libc functions in a global constructor. This improve
error reporting for the user, by only emiting a single warning for each
failed symbol lookup.

Change-Id: I47504846e44a68366870b983ff556158e634cf83
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>