Remove duplicates in PATH and CLASSPATH to fix bug 1401216

In sqf/sqenvcom.sh :
1. Removed duplicates from environment variables $CLASSPATH and $PATH,
   so sqenv.sh can be invoked repeatedly without them growing.
2. Removed test for JAVA_HOME containing exactly "1.7.0_67", which should
   help fix bug 1387238.
3. To reduce the places the recommended jdk version "1.7.0_67" appears,
   set this variable earlier:
     REQ_JDK_VER="1.7.0_67"
   This is used only in setting JAVA_HOME if it does not exist.
4. Removed jdk 1.6 lines since we stopped using that when directory
   sql/hbaseExtensions was removed about the end of September.
5. Made indentation more consistent.
6. Made formatting of "if" and "do" more consistent.
7. Used double-brackets in conditionals, which is safer as
   documented in http://mywiki.wooledge.org/BashFAQ/031
8. Use $(..) for subshells instead of less readable backticks.

In sqf/tools/sqtools.sh, added function remove_duplicates_in_path which
removes second and later appearances of the same value.

Change-Id: Iec48874b14d08ca8155006ab6a6a3f41c1714ca8
Merge "Remove duplicates in PATH and CLASSPATH to fix bug 1401216"