Only accept exact matches for selection keywords

Only accept exact matches for selection keywords

The selection parser tried to be nice to the user and also accept
unambiguous prefixes of keywords, but this also has a lot of side
effects that can be confusing (e.g., it was impossible to create
variables that had names that were prefixes to keywords or to other
variable names).  Additionally, this was the only case where user input
could cause an exception during tokenization of the input string, and
that wasn't handled very well during interactive input, either (it
caused the whole program to stop, instead of just reporting the error
like is done for other parsing errors).

Remove the logic, and only make the parser accept exact matches.
Add a few synonyms for keywords where there is a natural abbreviation.

Change-Id: I6041baa2f5a3b7dab87c3d5991e883d2d74ace66