Updated dosbox package fixes security vulnerabilities

Dosbox 0.74-3 is a security release: * Fixed that a very long line inside a bat file would overflow the parsing buffer. (CVE-2019-7165 by Alexandre Bartel) * Added a basic permission system so that a program running inside DOSBox can’t access the contents of /proc (e.g. /proc/self/mem) when / or /proc were (to be) mounted. (CVE-2019-12594 by Alexandre Bartel) It also brings several other fixes for out of bounds access and buffer overflows, and some fixes to the OpenGL rendering. The game compatibility should be identical to 0.74 and 0.74-2. It is recommended to use config -securemode when dealing with untrusted files.