BUG/MAJOR: ensure that hdr_idx is always reserved when L7 fetches are used

BUG/MAJOR: ensure that hdr_idx is always reserved when L7 fetches are used

Baptiste Assmann reported a bug causing a crash on recent versions when
sticking rules were set on layer 7 in a TCP proxy. The bug is easier to
reproduce with the "defer-accept" option on the "bind" line in order to
have some contents to parse when the connection is accepted. The issue
is that the acl_prefetch_http() function called from HTTP fetches relies
on hdr_idx to be preinitialized, which is not the case if there is no L7
ACL.

The solution consists in adding a new SMP_CAP_L7 flag to fetches to indicate
that they are expected to work on L7 data, so that the proxy knows that the
hdr_idx has to be initialized. This is already how ACL and HTTP mode are
handled.

The bug was present since 1.5-dev9.