This patch fixes compiler warnings like the following:
./lib/ovsdb-types.h:171:5: error: comparison of unsigned expression
>= 0 is always true [-Werror=type-limits]
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.
