Define prototype for exit and ensure references

Define a prototype for exit in stand.h. Provide a reference to exit in
a few conf.c files to ensure that its definition gets pulled in early.
Since exit() is a MD routine, it isn't defined in libsa. However,
libsa tends to be listed last and will soon have panic() in it which
calls exit(). The reference to exit early ensures that the MD exit is
available to satisfy linking for static libraries.