clvmd: Fix client list corruption

clvmd: Fix client list corruption

Centralise editing of the client list into _add_client() and
_del_client().  Introduce _local_client_count to track the size of the
list for debugging purposes.  Simplify and standardise the various ways
the list gets walked.

While processing one element of the list in main_loop(),
cleanup_zombie() may be called and remove a different element, so make
sure main_loop() refreshes its list state on return.  Prior to this
patch, the list edits for clients disappearing could race against the
list edits for new clients connecting and corrupt the list and cause a
variety of segfaults.

An easy way to trigger such failures was by repeatedly running shell
commands such as:
  lvs &; lvs &; lvs &;...;killall -9 lvs; lvs &; lvs &;...

Situations that occasionally lead to the failures can be spotted by
looking for 'EOF' with 'inprogress=1' in the clvmd debug logs.