Return -EINVAL from _from_string() functions

Return -EINVAL from _from_string() functions

We'd return -1 (-EPERM), even though we have a general rule to use real errno
values. The particular case that caught my attention was:

$ sudo udevadm control -l asdf
Failed to parse log priority 'asdf': Operation not permitted

... but "git grep 'r =.*_from_string' src/" return 110 hits. Confusingly, some
of the _from_string functions already return a proper errno value, so not all
of those are broken, but probably quite a few.