Fix two issues related to check-man

First, we don't need to check man pages at sed time, because this means
everyone building the package will do - we only need to check at docbook
time, which is mostly at developer time.

Second, don't force LC_ALL to C, as this breaks newer man-db. I've
verified and removing LC_ALL works fine across etch, hardy, lenny aid
squeeze/sid.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>