IPVS: Integrate ESP protocol into ip_vs_proto_ah.c

Rename all ah_* functions to ah_esp_* (and adjust comments). Move ESP
protocol definition into ip_vs_proto_ah.c and remove all usage of
ip_vs_proto_esp.c.

Make the compilation of ip_vs_proto_ah.c dependent on a new config
variable, IP_VS_PROTO_AH_ESP, which is selected either by
IP_VS_PROTO_ESP or IP_VS_PROTO_AH. Only compile the selected protocols'
structures within this file.

Signed-off-by: Julius Volz <juliusv@google.com>
Signed-off-by: Simon Horman <horms@verge.net.au>