Change references to INET, INET4, INET6, WNET and XNET to lowercase

[mrotteveel]

The current documentation occasionally uses uppercase INET, INET4, INET6, WNET and XNET for the protocol, but in practice Firebird only accepts lowercase inet, inet4, inet6, wnet and xnet as the protocol name (i.e. it's case-sensitive, not case-insensitive). Revise the various docs to consistently use lowercase.