add `__wasilibc_reset_preopens` (#489)

add `__wasilibc_reset_preopens` (#489)

This resets the preopens table to an uninitialized state, forcing it to be
reinitialized next time it is needed.  This is useful when pre-initializing
using e.g. `wizer` or `component-init`.  Such tools are capable of taking a
snapshot of a running application which may later be resumed on an unrelated
runtime (which may have its own, unrelated preopens).

Signed-off-by: Joel Dice <joel.dice@fermyon.com>