clean up cbt shell script syntax

[megri]

Makes cbt adhere more to the standard rules of shell scripts.

This commit

• quotes identifiers inside test-blocks ([ … ])
• replaces [ "$x" == "$y" ] with [ "$x" = "$y" ]
• replaces [ "$x" == "" ] with [ -z "$x" ]
• replaces [ "$x" != "" ] with [ -n "$x" ]
• replaces [ ! "$x" -eq "$y" ] with [ "$x" -ne "$y" ]

Additional changes

• replaces [ -f "$CBT_LOOP_FILE/_KILL_FILE" ] with [ -s "$CBT_LOOP_FILE/_KILL_FILE" ] to check if the file exists and has a non-zero size in an attempt to avoid looping over empty lists.

[CLAassistant]

All committers have signed the CLA.

[megri]

Seems like I have to sign the CLA every time I push something... weird. Fixed