allow to read environment variables as config parameters

Environment variables must be prefixed with winix_ to be loaded by winix.
The winix_ prefix is then removed and the key value converted to lowercase.

Sample:
evn WINIX_MY_OPTION=TEST123test winix -c config_file

This will add my_option to the config with value TEST123test.
Environment variables are read last so they will overwrite the values
from the configuration files.

winixd/core/config.cpp

@@ -95,8 +95,6 @@ bool Config::ReadConfig(const std::wstring & config_file)
 
 	if( status == pt::SpaceParser::ok )
 	{
-		AssignValues();
-		SetAdditionalVariables();
 		return true;
 	}
 	else
@@ -107,6 +105,12 @@ bool Config::ReadConfig(const std::wstring & config_file)
 }
 
 
+void Config::AssignValuesFromSpace()
+{
+	AssignValues();
+	SetAdditionalVariables();
+}
+
 
 
 void Config::AssignValues()

winixd/core/config.h

@@ -854,6 +854,8 @@ public:
 
 	Config();
 	bool ReadConfig(const std::wstring & config_file);
+	void AssignValuesFromSpace();
+
 
 	/*
 	 *