9pserve: msize not optimal

[vestein463]

if started without -m option, a message size of 8192 is used. Therefore the default blocksize is 8192-24, which is not a power of 2, leading to overhead in disk sector operations.

The default msize should be changed to 8192+IOHDRSZ = 8216. The man page should adjusted, too, either by change to the program by adding IOHDRSZ to the intended buffer size or explaining that IOHDRSZ should be added to the m value.