install: create_symlink() check unlink() return value

install: create_symlink() check unlink() return value

create_symlink() do not check the return value of unlink(), this may
confuse the user.

Before the unlink() call we check the 'force' argument. If it is not set
we fail with -EEXIST, otherwise we unlink() the file, therefore the next
symlink() should not fail with -EEXIST (do not count races...).

However since callers may not have appropriate privileges to unlink()
the file we lose the -EPERM or any other errno code of unlink(), and
return the -EEXIST of the next symlink(). Fix this by checking unlink()
results.

Before:
$ systemctl --force --root=~/container-03 set-default multi-user.target
Failed to set default target: File exists

After:
$ systemctl --force --root=~/container-03 set-default multi-user.target
Failed to set default target: Permission denied