session: process environment.d and allow empty variables

session: process environment.d and allow empty variables

1. All '*.env' files in an 'environment.d' directory alongside each
   potential 'environment' file will be parsed and added to the
   environment.

2. For the purposes of configuration merging, an environment definition
   exists at one level if either the 'environment' file is defined or
   its corresponding 'environment.d' contains any valid '*.env' file.

3. Variable declarations of the form "VARIABLE=", with no following
   value, will be written to the environment as empty strings.