Support for RELAX NG used to be enabled together with XML Schema support
(--with-schemas). Now there's a separate option and a new feature macro
LIBXML_RELAXNG_ENABLED.
Move source code for xmllint shell to shell.c and move it from the
libxml2 library to the xmllint executable.
Also allow shell to run without XPath and debug modules.
Add stubs for old shell API functions in legacy build mode.