XIA/Serval: rename `srvcid' to `serval'

This change matches userland label.
XIA/Serval: address CHECKSUM_PARTIAL bug

XIA Serval wasn't sending out the SYN packet when the outgoing
network device supported TCP checksumming in hardware.
Depending on the network card drive, the kernel could panic.
The e1000 card in my machine issued a friend warning in dmesg:

[ 2700.769728] e1000 0000:00:03.0 eth0: checksum_partial proto=dec0!

According to a comment in original Serval's
src/stack/serval_tcp.c:serval_tcp_sendpage(),
Serval cannot use this hardware feature due to the hardware
only supporting IP as the underlying protocol of TCP.

Besides fixing the cited bug, this patch cleans a little bit
around the code related to the constant CHECKSUM_PARTIAL.

See Cody's bug report at
https://github.com/AltraMayor/XIA-for-Linux/issues/3 for details.