read firstline: only map ENOENT to undef, raise error otherwise

read firstline: only map ENOENT to undef, raise error otherwise

Errors like permission denied or I/O ones should bubble up, otherwise
it might hide serious issues and seemingly continue to work, with a
wrong state or the like.

One could argue that the case for not existent should return undef,
while an empty file should return an empty string, but for that we
might want to check all use-sites first.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>