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.
MINOR: systemd wrapper: improve logging

Use standard error for logging messages, as it seems that this gets
messages to the systemd journal more reliably. Also use systemd's
support for specifying log levels via stderr to apply different levels
to messages.
MINOR: systemd wrapper: propagate exit status

Use HAProxy's exit status as the systemd wrapper's exit status instead
of always returning EXIT_SUCCESS, permitting the use of systemd's
`Restart = on-failure' logic.