exit(3) is implemented by the runtime and performs a number of shutdown actions before ultimately calling _exit(2) to terminate the program. We historically named the syscall table entry `exit` rather than `_exit`, but this requires special handling in libc/libsys to cause the `_exit` symbol to exist while implementing `exit` in libc. Declare the syscall as `_exit` and flow that through the system. Because syscall(SYS_exit, code) is fairly widely used, allow a configured extra line in syscall.h to define SYS_exit to SYS__exit. I've found no external uses of __sys_exit() so I've not bothered to create a compatability version of this private symbol. Reviewed by: imp, kib, emaste Differential Revision: https://reviews.freebsd.org/D51672