Harden pluggable transport exes shipped with Tor Browser

Right now we have something like

Checking obfs4proxy.exe for /DYNAMICBASE... FAIL
Checking obfs4proxy.exe for /NXCOMPAT... FAIL
Checking obfs4proxy.exe for /SAFESEH... PASS
Checking /obfs4proxy.exe ImageBase (0x400000 < 4GB)... FAIL

for 64bit versions and a similar output for 32bit ones. We should get rid of the FAILs.