docs: fix libkrun.h self-containment and net API refs

[immanuwell]

libkrun.h was not self-contained, so a normal C file that only includes the public header failed on uid_t and gid_t.
README also pointed to krun_add_net_unixdgram, but the exported API is krun_add_net_unixgram, so copy paste code just blows up.

Repro for the header bug:

#include "include/libkrun.h"
int main(void) { return 0; }

cc -std=c11 -Werror -I. /tmp/t.c -c

Repro for the README typo:

#include "include/libkrun.h"
#include <stdint.h>
int main(void) {
    uint8_t mac[6] = {0};
    return krun_add_net_unixdgram(1, 0, -1, mac, 0, 0);
}

This adds <sys/types.h>, fixes the README API name, and cleans the nearby unixgram docs. Small paper cut, but pretty real.