Revert "No need for usage()"

This reverts most of commit a6dc051e3744ce5b14c54d2d246d3e8258207e76 and fixes
some related stuff:

- keep spelling fixes from original commit
- make -h and -v also work when followed by more arguments
- any unknown flag prints usage
- fix output of -v to display "slock: version 1.3" instead of "slock: slock-1.3"

No need for usage()

There are 2 arguments why -v and -h are broken:

	1) if you are running off git, -v will show the last stable
           release, effectively making this option useless.
           people running stable versions leave open an attack surface
           this way in case there are vulnerabilities found.
           99% of the people are also using package managers to keep
           their software up to date, instead of running $TOOL -v to
           check how old it is.
        2) -h is a sad excuse for not just looking at the manual page
           (man 1 slock). Given we accept a post_lock_command, we can't
           be as liberal and just intercept certain flags.

I changed the manpage to reflect this change.

add slock.1 man page