Add printf(1)

This is a particularly interesting program.
I managed to implement everything according to POSIX except how
octal escapes are specified in the standard, which is yet another
format compared to the one demanded for tr(1).
This not only confuses people, it also adds unnecessary cruft
for no real gain.
So in order to be able to use unescape() easily and for consistency,
I used our initial format \o[oo] instead of \0[ooo].

Marked as optional is UTF-8 support for %c in the POSIX specification.
Given how well-developed libutf has become, doing this here was more
or less trivial, putting us yet again ahead of the competition.

README

@@ -51,7 +51,7 @@ The following tools are implemented ('*' == finished, '#' == UTF-8 support,
 =* nohup           yes                          none
 #* paste           yes                          none
 =* printenv        non-posix                    none
-   printf          stolen                       stolen
+#* printf          yes                          none
 =* pwd             yes                          none
 =  readlink        non-posix                    none
 =* renice          yes                          none