ensure function declarations are compiled with their definitions

ensure function declarations are compiled with their definitions

Function prototypes should be included when compiling their
definitions so that it is clear to compilers and static
analyzers that they are not static.

This means that several declarations have to be moved to the
related header files which in turn have to be included by the
source files implementing them.

Generally speaking this also improves the coding style and
makes this code more consistent with the rest that already
follows this rule.

Cc: Steffan Karger <steffan@karger.me>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20171111161836.23356-3-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15820.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>