Add file access tests using effective UID/GID (20.08)

[timopollmeier]

What:
The new functions gvm_file_exists, gvm_file_is_executable and
gvm_file_is_readable are added as an alternative to g_file_test that
tests the file access according to the effective user and group ID
instead of the real one.

Why:
The change is required for gvmd to work properly with the file flags
to set the UID and GID

How:
I tested the functions with a small test program like this:

#define _GNU_SOURCE

#include <stdio.h>
#include <unistd.h>
#include <gvm/util/fileutils.h>

int main ()
{
  const char *fname = "./test.txt";
  printf ("File: %s\n", fname);
  printf (" - exists: %d\n", gvm_file_exists (fname));
  printf (" - readable: %d\n", gvm_file_is_readable (fname));
  printf (" - executable: %d\n", gvm_file_is_executable (fname));
  printf ("eUID: %d, eGID: %d\n", geteuid(), getegid());
}

The cases I checked were:

• the file test.txt not existing
• the user running the test executable (without any special flags), owning the text file
• the user running the not having any permissions for text file
• running the executable as a user that normally has no permissions for the text file
  but setting the setuid and setgid flags of the executable so it is run as a user with
  access to the file.

Checklist:

• Tests
• CHANGELOG Entry