Various fixes for -Wconversion errors

Various fixes for -Wconversion errors

These are all fixes I considered "safe". They either

- Have sufficient checks/shifts for a cast to be safe
- Fix the type of a variable without requiring code changes
- Are in non-critical unittest code

v2:
 - add min_size instead of abusing min_int
v6:
 - remove change of return value of link_socket_write.
   Move to separate patch.

Change-Id: I6818b153bdeb1eed65870af99b0531e95807fe0f
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20240910122008.23507-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29172.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>