login-common: Rename sasl-server.[ch] to sasl-proxy.[ch]

login-common: Rename sasl-server.[ch] to sasl-proxy.[ch]

Both lib-sasl and login-common had a sasl-server.h and both were installed
into $(pkgincludedir), so one overwrote the other. They also shared the same
SASL_SERVER_H include guard, and since login-common builds with -I on
lib-sasl, the unqualified #include "sasl-server.h" was ambiguous.

Rename the login-common header and source and change the symbol prefix from
sasl_server_ to sasl_proxy_. The login process doesn't implement SASL itself,
it only proxies the SASL exchange between the client and the auth process.