BUG/MINOR: init: relax LSTCHK_NETADM checks for non root

BUG/MINOR: init: relax LSTCHK_NETADM checks for non root

Linux capabilities support and ability to preserve it for running process
after switching to a global.uid was added recently by the commit bd84387beb26
("MEDIUM: capabilities: enable support for Linux capabilities")).
This new feature hasn't yet been taken into account by last config checks,
which are performed at initialization stage.

So, to update it, let's perform it after set_identity() call. Like this,
current EUID is already changed to a global.uid and prepare_caps_for_setuid()
would unset LSTCHK_NETADM flag, only if capabilities given in the 'setcap'
keyword in the configuration file were preserved.

Otherwise, if system doesn't support Linux capabilities or they were not set
via 'setcap', we keep the previous strict behaviour: process will terminate
with an alert, in order to insist that user: either needs to change
run UID (worst case: start and run as root), or he needs to set/recheck
capabilities listed as 'setcap' arguments.

In the case, when haproxy will start and run under a non-root user this patch
doesn't change the previous behaviour: we'll still let him try the
configuration, but we inform via warning that unexpected things may occur.

Need to be backported until v2.9, including v2.9.