More slab allocator updates.

src/alloc-slab.c: Disable extra debugging by default.  Add more
statistics counters.  Change element type of little_slabs[].  Keep a
global list of unused (full) slab pages, rather than per-slab.
Various other cleanups and fixes.

src/chanserv.h: Can only be off-channel with off_channel > 1.

src/main.c: Update years in copyright notice.

src/opserv.c: Remove OSMSG_STATS_MEMORY.  Make cmd_stats_memory()
allocator-specific.
git-archimport-id: srvx@srvx.net--2005-srvx/srvx--devo--1.3--patch-12