Fix types around buffer_list_push(_data)

Fix types around buffer_list_push(_data)

In C, strings are char pointers, not unsigned char pointers.  And
arbitrary data is represented by a void pointer.  Change buffer_list_push
and buffer_list_push_data to follow these rules, and remove any now
unneeded casts.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1515573259-20968-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16186.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit b395f36e578b2def9da8e9347c0afa79814c0c7d)