MINOR: systemd: Check configuration before start

Adds a configuration check before starting the haproxy service.
(cherry picked from commit 91a9f1a2e1421a376a4b63f9342addd59cb2b244)
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.
(cherry picked from commit e1cfc1f2b469fb86b9826ff12cdd6a24a1f3ded0)
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.
(cherry picked from commit fad4ffc89337277f3d5ed32b66986730e891558a)
[RELEASE] Released version 1.5.6

Released version 1.5.6 with the following main changes :
    - BUG/MEDIUM: systemd: set KillMode to 'mixed'
    - MINOR: systemd: Check configuration before start
    - BUG/MEDIUM: config: avoid skipping disabled proxies
    - BUG/MINOR: config: do not accept more track-sc than configured
    - BUG/MEDIUM: backend: fix URI hash when a query string is present