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
e7e964cb2e