let printf understands %c

[AnzhiZhang]

We utilize xv6 as part of our teaching materials. Recently, we designed an exercise in which students use pipes to send bytes and display characters via printf. During this, we observed that the current printf implementation in xv6 does not support the %c specifier to print single characters.

This update involves only two additional lines and allows for %c usage, which is frequently needed for character display. We believe this adjustment would enhance the learning experience and minimize confusion among students.

It looks like forgot to implement when rewriting printf in dd2574b. In the commented parts, parsing %c still exists.

resolves #283