Use 'unsigned char' rather than 'char' for pchMessageStart.

Regarding https://bitcointalk.org/index.php?topic=28022.0

main.cpp has: "char pchMessageStart[4] = { 0xf9, 0xbe, 0xb4, 0xd9 };"
Per discussion on the thread linked, leaving the signedness of
pchMessageStart is unsafe for values > 0x80. This patch specifies
'unsigned char' in main.cpp and net.h.

Signed-off-by: Jeff Garzik <jgarzik@pobox.com>