(gud-jdb-sourcepath): New variable, saves jdb -sourcepath parameter value.

(gud-jdb-sourcepath): New variable, saves jdb -sourcepath parameter value.
(gud-jdb-build-source-files-list): Comment clarification.
(gud-jdb-massage-args): Reworked into loop-based argument list
processing in order to support -classpath and -sourcepath argument processing.
(gud-jdb-find-source-using-classpath): Prepend gud-jdb-sourcepath
to gud-jdb-classpath to obtain search list.
(gud-jdb-parse-classpath-string): Remove any trailing slashes from
directory names in classpath/sourcepath lists, update comment.
(jdb): Add setting of gud-jdb-sourcepath, update comment.
(gud-find-class): Add gud-jdb-sourcepath use, and correct
behaviour for the case when classpath is not used--backward
compatibility fix.