MINOR: cfgparse: try to suggest correct variable names on errors

MINOR: cfgparse: try to suggest correct variable names on errors

When an empty argument comes from the use of a non-existing variable,
we'll now detect the difference with an empty variable (error pointer
points to the variable's name instead), and submit it to env_suggest()
to see if another variable looks likely to be the right one or not.

This can be quite useful to quickly figure how to fix misspelled variable
names. Currently only series of letters, digits and underscores are
attempted to be resolved as a name.