Change type of frame.mss_fix to uint16_t

Change type of frame.mss_fix to uint16_t

Since in the end this always ends up as an uint16_t
anyway, just make the conversion much earlier. Cleans
up the code and removes some -Wconversion warnings.

v2:
 - proper error handling in options.c
v4:
 - also introduce a minimum mssfix

Change-Id: Id8321dfbb8ad8d79f4bb2a9da61f8cd6b6c6ee26
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Message-Id: <20231009105151.34074-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/search?l=mid&q=20231009105151.34074-1-frank@lichtenheld.com
Signed-off-by: Gert Doering <gert@greenie.muc.de>