Check installer permissions using geteuid

[rilysh]

Hi!
I noticed EBPF doesn't correctly check the permission whether the user has root privilege or not. Instead, it relies on other functions' errors. (E.g. In libsysinternalsEBPFinstaller.c at line 279, it relies on mkdir(...) and stat(...) functions' success/errors). It would be better to just check the permission through geteuid(...) instead.

if (geteuid() != (uid_t)0) {
     // Error message
}

Something like this also fixes an issue where with the -u flag (uninstall), the installer (libsysinternalsEBPFinstaller.c) doesn't check the permission at all.