Implement environment variable interpolation in defaults files.

This allows the syntax `${HOME}` to be used, in fields that expect
file paths only.  Any environment variable may be interpolated
in this way. A warning will be raised for undefined variables.
The special variable `USERDATA` is automatically set to the
user data directory in force when the defaults file is parsed.
(Note: it may be different from the eventual user data directory,
if the defaults file or further command line options change that.)

Closes #5982.
Closes #5977.
Closes #6108 (path not taken).

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