Improve LXC detection

[opty77]

https://github.com/troglobit/sysklogd/blob/1803a86edc1605d5cca3e4e2e39a4659607b1aef/src/syslogd.c#L250

needn't always work (e.g. syslogd restart in terminal or running under unprivileged user).

Slackware way seems more reliable.

[opty77]

Quick sample:

// [...]
#include <stdio.h>
#include <stdlib.h>

#include <errno.h>
#include <string.h>

void main(int argc, char **argv)
{
    char *environ_file = "/proc/1/environ";
    FILE *environ = fopen(environ_file, "r");
    char *arg = 0;
    size_t size = 0;
    char *tok = 0;

    if (!environ) {
        printf("Cannot open %s: %s\n", environ_file, strerror(errno));
        return;
    }

    while (getdelim(&arg, &size, 0, environ) != -1) {
        tok = strtok(arg, "=");
        if (tok && !strcmp(tok, "container")) {
            puts(strtok(NULL, "="));
            break;
        }
    }
    free(arg);
    fclose(environ);
}

[opty77]

Regarding https://github.com/troglobit/sysklogd/issues/76#issuecomment-2156719868, improving doesn't seem worth the effort but YMMV again.