Audit nl(1)

1) Refactor the manpage to use the num-syntax and concise wording.
2) Build format instead of having a list of static strings.
3) BUGFIX: if (!buf[0] || buf[0] == '\n') Process last-read-line
           properly.
4) BUGFIX: In case we hit a formatting line, print a newline instead
           of just dropping it.
5) Use a switch instead of having spaghetti-cases.
6) Don't use printf-magic but explicitly do a putchar(' ')-loop.
7) Update usage(), indent properly.
8) BUGFIX: strchr is not NULL when type[0] is \0. Check for \0
           separately beforehand.
9) Reorder arg.h-cases for better readability.

README

@@ -49,7 +49,7 @@ The following tools are implemented ('*' == finished, '#' == UTF-8 support,
 =*| mktemp          non-posix                    none
 =*| mv              yes                          none (-i)
 =*| nice            yes                          none
-#*  nl              yes                          none
+#*| nl              yes                          none
 =*| nohup           yes                          none
 #*| paste           yes                          none
 =*| printenv        non-posix                    none