Portable mode fix
Portable mode fix

The option should not be available if the directory with the executable
is not writable, such as when running from a dmg on OSX.

hb9kns/PyBitmessage,bmng-dev/PyBitmessage,torifier/PyBitmessage,timothyparez/PyBitmessage,debguy0x/PyBitmessage,hb9kns/PyBitmessage,torifier/PyBitmessage,bmng-dev/PyBitmessage,debguy0x/PyBitmessage,timothyparez/PyBitmessage,debguy0x/PyBitmessage,torifier/PyBitmessage,bmng-dev/PyBitmessage,debguy0x/PyBitmessage,hb9kns/PyBitmessage,timothyparez/PyBitmessage,hb9kns/PyBitmessage,timothyparez/PyBitmessage,torifier/PyBitmessage