ovsdb: Require database, table, column names to be valid identifiers.

ovsdb: Require database, table, column names to be valid identifiers.

Database, table, and column names have always been required by the OVSDB
specification to be identifiers (e.g. valid C identifiers), but this has
never been enforced.

This commit adds enforcement and fixes one instance of an invalid column
name in the vswitch schema.