Motivated by https://gitlab.com/uplex/varnish/slash/-/issues/41#note_1688912442

Also added to Varnish-Cache: https://github.com/varnishcache/varnish-cache/commit/24b434383c616639d5aa9be9b5ba3647a418d64c

Fixes #42

to enable parallel tests, for example:

$ for i in {1..24} ; do while ./src/fellow_cache_test /tmp/f.${i} >/dev/null 2>/dev/null ; do : ;done & done ; wait

Motivated by #42

Have seen (fbo) != NULL in fellow_cache_async_write_complete():

 #14 0x00007f9aa30957e5 in fellow_cache_async_write_complete (fc=0x7f9aa2c41300, ptr=0x7f9a9ff4df58, result=4096)
    at fellow_cache.c:2791
 #15 0x00007f9aa3096403 in fellow_cache_seg_async_compl_cb (priv=0x7f9aa2c41300, status=0x7f9a999fa3e0, n=1)
    at fellow_cache.c:2951

(gdb) info local
fbio = 0x7f9a9ff4df58
fbo = 0x0
fcs = 0x0
fco = 0x7f9a9ff4f000
fcos_next = FCOS_INVAL
type = FBIO_SEG
io_outstanding = 2 '\002'
refcount = 0
__PRETTY_FUNCTION__ = "fellow_cache_async_write_complete"
lcb = {{magic = 2863944409, n_add = 0, l_rem = 2, n_rem = 0, fco = 0x7f9a9ff4f000, add = {vtqh_first = 0x0,
      vtqh_last = 0x7f9a999fa2b8}, fcs = 0x7f9a9ff4df5c}}
__func__ = "fellow_cache_async_write_complete"
_pterr281611 = <optimized out>
_pterr282913 = <optimized out>
_pterr289715 = <optimized out>
(gdb) p *fbio
$1 = {magic = 3019, retries = 0, type = FBIO_SEG, sync = FBIOS_ASYNC, fbo = 0x0, u = {fcs = 0x0, seglist = {fdsl = 0x0,
      reg = {off = 0, size = 0}}}}

we can have FCS_BUSY segments before OBJ_ITER_END while streaming.

At the last segment, do not advance to the next segment list if it is
still empty.

we only jump to the again label if we did not get a reference.

Motivated by #40

CID#469253

CID#469261

Coverity CID#469242

Coverity CID#469229

To address some Coverity pedentry with minor impact (at most)

CID#469230

Coverity CID#469233

Spotted by Coverity CID#469228, but it is irrelevant because only in
test code

Good catch by Coverity CID#469254

Coverity CID#469252

Ref Coverity CID#469262

The optimized case for multiple segments from the same fco did
not work as expected, continue did not continue the inner loop.

Spotted by Coverity, CID#469236

Coverity CID#469225

Ref CID#469268

Motivated by #41

the disk object is always a multiple of 4K, and for higher values (like
12K), rounding up to the next power of two does not make sense.

So, just use two allocations for FCO and FDO always.

For now, this uses a local installation of coverity's tools in the slink
subfolder.