Basic compilation fixes.
Remove a usage of free()
Fix most of the --enable-gcc-warnings warnings in the sandbox code
Fix a warning related to SCMP_CMP definition in header.

SCMP_CMP(a,b,c) leaves the fourth field of the structure undefined,
giving a missing-initializer error.  All of our uses are
three-argument, so I'm overriding the default.
Split libevent AC_CHECK_FUNCS call into multiple lines
Build correctly with older libevents
Do not try to add non-existent syscalls.
Fix compilation on OSX
Fix check-spaces