MEDIUM: server: introduce srv_alloc()/srv_free() to alloc/free a server

MEDIUM: server: introduce srv_alloc()/srv_free() to alloc/free a server

It happens that we free servers at various places in the code, both
on error paths and at runtime thanks to the "server delete" feature. In
order to switch to an aligned struct, we'll need to change the calloc()
and free() calls. Let's first spot them and switch them to srv_alloc()
and srv_free() instead of using calloc() and either free() or ha_free().
An easy trap to fall into is that some of them are default-server
entries. The new srv_free() function also resets the pointer like
ha_free() does.

This was done by running the following coccinelle script all over the
code:

  @@
  struct server *srv;
  @@
  (
  - free(srv)
  + srv_free(&srv)
  |
  - ha_free(&srv)
  + srv_free(&srv)
  )
  @@
  struct server *srv;
  expression e1;
  expression e2;
  @@
  (
  - srv = malloc(e1)
  + srv = srv_alloc()
  |
  - srv = calloc(e1, e2)
  + srv = srv_alloc()
  )

This is marked medium because despite spotting all call places, we can
never rule out the possibility that some out-of-tree patches would
allocate their own servers and continue to use the old API... at their
own risk.