Do not define xcache_unset_by_prefix() if it does not exist.

The defined function is not compatible with the function provided by xcache
because it does not honor the prefix parameter. Thus defining it like this is
a bad idea.
Allow usage of xCache variable cache if xcache_unset_by_prefix() is present.
Clear xCache OpCode cache instead of variable cache in clearOpcodeCache().
xcache_unset_by_prefix() returns feedback, return it.
Add link to XCache API in class documentation.
xCache -> XCache
Make it clear that log message is about the XCache opcode cache.
Do not mention php.ini, it may be defined in xcache.ini or so.