innodb_additional_mem_pool_size, because nothing will
be allocated from mem_comm_pool.

mem_pool_create(): Remove the assertion about size.  The function will
work with any size.  However, an assertion would fail in ut_malloc_low()
when size==0.

mem_init(): When srv_use_sys_malloc is set, pass size=1 to mem_pool_create().

mem0mem.c: Add #include "srv0srv.h" that is needed by mem0dbg.c.