Move file-related functions from misc.c to platform.c

Move file-related functions from misc.c to platform.c

To avoid having to include misc.c - which is a dependency mess - in the
tls-crypt unit tests, move file-handing related functions to platform.c
(which is where other file-related functions already reside).

Note that platform_create_temp_file() needs random.  To avoid including
misc.c in other tests that use platform.c, add a mock get_random().

(Almost every test includes platform.c, because buffer.c depends on it.
That smells like it needs cleanup too, but not in this patch set.)

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20180704175404.22371-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17208.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>