doc: Fix links in "Further help and development"

Hyperlink the mailing list to the proper info page.

Tweak the GitHub link to use https.
doc: Fix docs on $HOME/$USER

The docs claimed that the $HOME and $USER variables could only be
changed by the root user. This is untrue. They can be changed by
non-root users as well.
doc: Document how IFS affects command substitution

IFS is used for more than just the read builtin. Setting it to the empty
string also disables line-splitting in command substitution, and it's
done this for the past 7 years. Some day we may have a better way to do
this, but for now, document the current solution.
Clean up the IFS handling in command substitution

Remove the useless ASCII test of the first byte of IFS. We don't split
on the first character, we only use a non-empty IFS as a signal to split
on newlines.