Just like the last commit, but now for manpages.
genMan still works with a buffer and returns []byte instead of working
directly with an io.Writer. This is because, in turn, md2man takes byte
slices instead of readers and writers.
Wrapping genMan around a writer is unnecessary especially since it's not
an exported function, and also because we'd still need a buffer to get
the output bytes.
5df1341f93