common: -- support for env_to_vec

When CEPH_ARGS is parsed each side of the -- must be appended to the
corresponding side of the existing argument list. For instance when

   -a -b -- foo bar

is merged with a CEPH_ARGS containing

   -c -d -- frob nitz

it must become

   -a -b -c -d -- foo bar frob nitz

http://tracker.ceph.com/issues/7578 fixes #7578

Signed-off-by: Loic Dachary <loic@dachary.org>