MINOR: systemd: Check configuration before start

Adds a configuration check before starting the haproxy service.
BUG/MINOR: config: do not accept more track-sc than configured

MAX_SESS_STKCTR allows one to define the number of stick counters that can
be used in parallel in track-sc* rules. The naming of this macro creates
some confusion because the value there is sometimes used as a max instead
of a count, and the config parser accepts values from 0 to MAX_SESS_STKCTR
and the processing ignores anything tracked on the last one. This means
that by default, track-sc3 is allowed and ignored.

This fix must be backported to 1.5 where the problem there only affects
TCP rules.
BUG/MEDIUM: backend: fix URI hash when a query string is present

Commit 98634f0 ("MEDIUM: backend: Enhance hash-type directive with an
algorithm options") cleaned up the hashing code by using a centralized
function. A bug appeared in get_server_uh() which is the URI hashing
function. Prior to the patch, the function would stop hashing on the
question mark, or on the trailing slash of a maximum directory count.
Consecutive to the patch, this last character is included into the
hash computation. This means that :

    GET /0
    GET /0?

Are not hashed similarly. The following configuration reproduces it :

    mode http
    balance uri
    server s1 0.0.0.0:1234 redir /s1
    server s2 0.0.0.0:1234 redir /s2

Many thanks to Vedran Furac for reporting this issue. The fix must
be backported to 1.5.
MINOR: ssl: use SSL_get_ciphers() instead of directly accessing the cipher list.