MINOR: systemd wrapper: re-execute on SIGUSR2

MINOR: systemd wrapper: re-execute on SIGUSR2

Re-execute the systemd wrapper on SIGUSR2 and before reloading HAProxy,
making it possible to load a completely new version of HAProxy
(including a new version of the systemd wrapper) gracefully.
Since the wrapper accepts no command-line arguments of its own,
re-execution is signaled using the HAPROXY_SYSTEMD_REEXEC environment
variable.

This is primarily intended to help seamless upgrades of distribution
packages.