parse: change offset type

to fit with mnl_nlmsg_get_payload_offset()
examples: set attr table
examples: fix trivial error message
seq, portid: change type to uint32_t

following struct nlmsghdr
examples: use mnl_nlmsg_get_payload()

not increment (nlmsghdr) pointer
document: mnl_socket_open

fix return value on error
Merge branch 'master' into mmap2