Quoting POSIX[0]:
"Care should be taken, also, to call _exit() rather than exit() if exec cannot be used, since
exit() flushes and closes standard I/O channels, thereby damaging the parent process' standard
I/O data structures. (Even with fork(), it is wrong to call exit(), since buffered data would
then be flushed twice.)"
[0]: http://pubs.opengroup.org/onlinepubs/009695399/functions/vfork.html
1) Shorten synopsis and reflect this in the manual
2) Use argv0 in usage()
3) Decrement argc in argv-loop for consistency
4) Make it clearer which error-code results from which errno in enprintf
5) Use idiomatic for-loop also for environ. Don't increment these pointers
in the loop itself!
