BUG/MEDIUM: systemd: set KillMode to 'mixed'

By default systemd will send SIGTERM to all processes in the service's
control group. In our case, this includes the wrapper, the master
process and all worker processes.

Since commit c54bdd2a the wrapper actually catches SIGTERM and survives
to see the master process getting killed by systemd and regard this as
an error, placing the unit in a failed state during "systemctl stop".

Since the wrapper now handles SIGTERM by itself, we switch the kill mode
to 'mixed', which means that systemd will deliver the initial SIGTERM to
the wrapper only, and if the actual haproxy processes don't exit after a
given amount of time (default: 90s), a SIGKILL is sent to all remaining
processes in the control group. See systemd.kill(5) for more
information.

This should also be backported to 1.5.
BUG/MEDIUM: config: avoid skipping disabled proxies

Paul Taylor and Bryan Talbot found that after commit 419ead8 ("MEDIUM:
config: compute the exact bind-process before listener's maxaccept"),
a backend marked "disabled" would cause the next backend to be skipped
and if it was the last one it would cause a segfault.

The reason is that the commit above changed the "while" loop for a "for"
loop but a "continue" statement still incrementing the current proxy was
left in the code for disabled proxies, causing the next one to be skipped
as well and the last one to try to dereference NULL when seeking ->next.

The quick workaround consists in not disabling backends, or adding an
empty dummy one after a disabled section.

This fix must be backported to 1.5.
MINOR: systemd: Check configuration before start

Adds a configuration check before starting the haproxy service.
Allows you to extract a field from an input string

This is useful when trying to dynamically choose the stickyness based
on the path, or the hostname.

For example,
stick on path extract_field(/,3)

Will return baz from the path /foo/bar/baz