Fix three embarassing thinkos in the new dmesg.c:

Fix three embarassing thinkos in the new dmesg.c:
1) the c argument shouldn't have had a : after that, dunno how that got there.
2) the xgetlarg for level was using size
3) because xgetlarg's error message _SUCKS_ (it does a show_usage() rather than giving any specific info about the range that was violated) I dropped the range down to 2 bytes.  (Which works fine, I dunno why we were nit-picking about that...)