For panics from the LRU thread, we have, so far, been stepping in the dark if we
did not have a core dump with the segment/object.

This commit adds a dummy struct busyobj to the lru thread with the only purpose
of providing the linkage for the Varnish-Cache panic handler to call our panic
handler to in turn dump information about the currently handled objcore.

Tested with the following patch ...

diff --git a/src/fellow_cache.c b/src/fellow_cache.c
index e82f49a1..464921aa 100644
--- a/src/fellow_cache.c
+++ b/src/fellow_cache.c
@@ -5253,6 +5253,8 @@ fellow_cache_lru_work(struct worker *wrk, struct fellow_cache_lru *lru)
                CHECK_OBJ_NOTNULL(fco, FELLOW_CACHE_OBJ_MAGIC);
                lru->panic_bo.fetch_objcore = fco->oc;
                assert(fco->lru == lru);
+               if (fco->oc != NULL)
+                       WRONG("test");
                r = pthread_mutex_trylock(&fco->mtx);
                if (r != 0) {
                        assert(r == EBUSY);

... vtc/fellow_b00062.vtc gives us information about the object as expected:

**** v1    CLI RX|thr.busyobj = 0x7f1835607088 {
**** v1    CLI RX|  end = (nil),
**** v1    CLI RX|  retries = 0,
**** v1    CLI RX|  ws = 0x7f18356070d0 {
**** v1    CLI RX|    .magic = 0x00000000 EXPECTED: WS_MAGIC=0x35fac554
**** v1    CLI RX|  }
**** v1    CLI RX|  ws_bo = (nil),
**** v1    CLI RX|  objcore[fetch] = 0x7f1835a0c000 {
**** v1    CLI RX|    refcnt = 1,
**** v1    CLI RX|    flags = {},
**** v1    CLI RX|    exp_flags = {refd},
**** v1    CLI RX|    exp = {1732892693.194625, 1.000000, 1.000000, 86400.000000},
**** v1    CLI RX|    objhead = 0x7f1835a05070,
**** v1    CLI RX|    stevedore = 0x7f1843ecf000 (fellow fellow) {
**** v1    CLI RX|      priv2 = 2179073, fco = 0x7f1840f0c000 {
**** v1    CLI RX|        logstate = 4, lru = 0x7f1835607000, ntouched = 0,
**** v1    CLI RX|        fcr = { ok { r.integer = 1089519616, r.ptr = 0x7f1840f0c000}},
**** v1    CLI RX|        fdo_fcs = 0x7f1840f0c0d8 {
**** v1    CLI RX|          state = FCO_INCORE, fcs_onlru = 1, fco_infdb = 1,
**** v1    CLI RX|          lcb_add_head = 0, lcb_add_tail = 0, lcb_remove = 0, fco_lru_mutate = 0,
**** v1    CLI RX|          refcnt = 1, fco = 0x7f1840f0c000,
**** v1    CLI RX|          disk_seg = 0x7f1840f08008 {
**** v1    CLI RX|            fht = 3, segnum = 0,
**** v1    CLI RX|            seg = {.off = 2179072, .size = 4096},
**** v1    CLI RX|            fh[0..8] = 8422c2ee75247f6f,
**** v1    CLI RX|          },
**** v1    CLI RX|          alloc = {.ptr = 0x7f1840f08000, .size = 4096},
**** v1    CLI RX|          union { .fcs_len = 139742145445888, .fco_fdo = 0x7f1840f08000 },
**** v1    CLI RX|        },
**** v1    CLI RX|        esidata = 0x7f1840f0c128 {
**** v1    CLI RX|          state = FCAA_USABLE, fcs_onlru = 0, fco_infdb = 0,
**** v1    CLI RX|          lcb_add_head = 0, lcb_add_tail = 0, lcb_remove = 0, fco_lru_mutate = 0,
**** v1    CLI RX|          refcnt = 0, fco = 0x7f1840f0c000,
**** v1    CLI RX|          disk_seg = 0x7f1840f080f0 {
**** v1    CLI RX|            fht = 3, segnum = 0,
**** v1    CLI RX|            seg = {.off = 0, .size = 0},
**** v1    CLI RX|            fh[0..8] = 0000000000000000,
**** v1    CLI RX|          },
**** v1    CLI RX|          alloc = {.ptr = (nil), .size = 0},
**** v1    CLI RX|          union { .fcs_len = 0, .fco_fdo = (nil) },
**** v1    CLI RX|        },
**** v1    CLI RX|        fcsl = 0x7f1840f0c168 {idx = 0, lsegs = 63, fcsl_sz 0, fdsl_sz = 0, fdsl_idx = 0, fdsl_lsegs = 63, fdsl_nsegs = 63 },
**** v1    CLI RX|        fcsl = 0x7f1840f4b000 {idx = 1, lsegs = 70, fcsl_sz 4096, fdsl_sz = 4096, fdsl_idx = 1, fdsl_lsegs = 70, fdsl_nsegs = 70 },
**** v1    CLI RX|        fcsl = 0x7f1840f93000 {idx = 2, lsegs = 70, fcsl_sz 4096, fdsl_sz = 4096, fdsl_idx = 2, fdsl_lsegs = 70, fdsl_nsegs = 70 },
**** v1    CLI RX|        fcsl = 0x7f1840fdb000 {idx = 3, lsegs = 53, fcsl_sz 3136, fdsl_sz = 4096, fdsl_idx = 3, fdsl_lsegs = 53, fdsl_nsegs = 53 },
**** v1    CLI RX|        fcsl = NULL
**** v1    CLI RX|      },
**** v1    CLI RX|    },
**** v1    CLI RX|  },
**** v1    CLI RX|  flags = {},
**** v1    CLI RX|  storage = (nil),
**** v1    CLI RX|  director_resp = director_req,
**** v1    CLI RX|  vcl[vcl] = NULL
**** v1    CLI RX|},
**** v1    CLI RX|

Motivated by #83