Fix multi-line comment regular expression to handle (non) nested comments.

Fix multi-line comment regular expression to handle (non) nested comments.

Ken reminded me of a couple cases that I should be testing. These are
the non-nestedness of things that look like nested comments as well as
potentially tricky things like "/*/" and "/*/*/".

The (non) nested comment case was not working in the case of the
comment terminator with multiple '*' characters. We fix this by not
considering a '*' as the "non-slash" to terminate a sequence of '*'
characters within the comment. We also fix the final match of the
terminator to use '+' rather than '*' to require the presence of a
final '*' character in the comment terminator.