Assorted code cleanups

• Remove unnecessary casts from malloc & free calls
• Variable scope reduction as recommended by cppcheck
• Mark two dpy parameters const as suggested by cppcheck
• Quiet -Wconditional-uninitialized warnings from clang
• Resolve 36 of 40 -Wextra-semi-stmt warnings from clang
• Handle 63 of 63 -Wshorten-64-to-32 warnings from clang
• Handle 60 of 60 -Wimplicit-int-conversion warnings from clang
• Handle 6 of 9 -Wsign-conversion warnings from clang
• XFixesCloseDisplay: Mark codes as unused