Expand environment variables in strings directly

Make "$FOO" directly reference the environment variable $FOO in e.g.
'source' statements, instead of the symbol FOO. Use os.path.expandvars()
to expand strings (which preserves "$FOO" as-is if no environment
variable FOO exists).

This gets rid of the 'option env' "bounce" symbols, which are mostly
just spam and are buggy in the C tools (dependencies aren't always
respected, due to parsing and evaluation getting mixed up). The same
change will probably appear soon in the C tools as well.

Keep accepting 'option env' to preserve some backwards compatibility,
but ignore it when expanding strings. For compatibility with the C
tools, bounce symbols will need to be named the same as the environment
variables they reference (which is the case for the Linux kernel).

This is a compatibility break, so the major version will be bumped to 6
at the next release.

The main motivation for adding this now is to allow recording properties
on each MenuNode in a clean way. 'option env' symbols interact badly
with delayed dependency propagation.

Side note: I have a feeling that recording environment variable values
might be redundant to trigger rebuilds if sync_deps() is run at each
compile. It should detect all changes to symbol values due to
environment variables changing value.

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