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.

src/basic/string-table.c

@@ -5,11 +5,11 @@
 
 ssize_t string_table_lookup(const char * const *table, size_t len, const char *key) {
         if (!key)
-                return -1;
+                return -EINVAL;
 
         for (size_t i = 0; i < len; ++i)
                 if (streq_ptr(table[i], key))
                         return (ssize_t) i;
 
-        return -1;
+        return -EINVAL;
 }