Use constructor functions for optional init routines. (#142)

Use constructor functions for optional init routines. (#142)

* Use constructor functions for optional init routines.

Instead of using weak symbols, use constructor function attributes for the
environment and preopen initialization routines. This is simpler, uses
less code, and is more LTO-friendly.

* Change the constructor priorities to start at 50.

We don't currently have specific plans for other levels in the reserved
range (0-100), so leave room for both lower and higher priorities.