Audit head(1)

1) Use (s)size_t in head().
2) BUGFIX: only check buf[len - 1] when len > 0, else there would
   be an overflow when getline returns 0 (which can happen) and a
   very potential segmentation fault.
3) fix error-messages.
4) update usage().
5) argv-argc-style.
6) clear up the main loop with if (newline).
7) add newline before return.

README

@@ -35,7 +35,7 @@ The following tools are implemented ('*' == finished, '#' == UTF-8 support,
 =   find            yes                          none
 #*  fold            yes                          none
 =*  grep            yes                          none
-=*  head            yes                          none
+=*| head            yes                          none
 =*| hostname        non-posix                    none
 =*  kill            yes                          none
 =*| link            yes                          none