maint: mark some _Noreturn functions

maint: mark some _Noreturn functions

* src/basenc.c (finish_and_exit, do_encode, do_decode):
* src/comm.c (compare_files):
* src/tsort.c (tsort):
* src/uptime.c (uptime):
Mark with _Noreturn.  Otherwise, unoptimized compilations may warn
that the calling renamed-main function doesn't return a value,
when !lint and when single-binary.