MINOR: map: Add payload support to "add map"

MINOR: map: Add payload support to "add map"

It is now possible to use a payload with the "add map" command.
These syntaxes will work the same way:

 # echo "add map #-1 key value" | socat /tmp/sock1 -

 # echo -e "add map #-1 <<\n$(cat data)\n" | socat /tmp/sock1 -

with

 # cat data
 key1 value1 with spaces
 key2 value2
 key3 value3 also with spaces

Signed-off-by: Aurélien Nephtali <aurelien.nephtali@corp.ovh.com>