Introduce buffer_write_file()

Introduce buffer_write_file()

Rewrite buf_write_string_file to buffer_write_file, which is simpler to
use and can deal with not-null-terminated strings.  Mostly implemented so
this can be easily reused for tls-crypt-v2 (client) key files.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Tested-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <1533542553-7383-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17371.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>