BUG/MINOR: cfgparse-tcp: relax namespace bind check

BUG/MINOR: cfgparse-tcp: relax namespace bind check

Commit 5cbb278 introduced cap_sys_admin support, and enforced checks for
both binds and servers. However, when binding into a namespace, the bind
is done before dropping privileges. Hence, checking that we have
cap_sys_admin capability set in this case is not needed (and it would
decrease security to add it).
For users starting haproxy with other user than root and without
cap_sys_admin, bind should have already failed.
As a consequence, relax runtime check for binds into a namespace.

(cherry picked from commit f0a07f834c001c5b505e84b0f0b103e530e87d1b)
Signed-off-by: Willy Tarreau <w@1wt.eu>