Avoid adding a bootclasspath if it already exists

[jvican]

This is ported from sbt/sbt: sbt/sbt#762 (comment).

In short: even if there already exists a bootclasspath flag, Zinc adds another bootclasspath entry with the Scala library, effectively disabling the bootclasspath specified by the user. See discussion in the original issue.

Zinc should check if this flag already exists, log that it does, and then assumes that the user is passing in the scala library in the boot classpath. The log shown to the user should explicitly mention this assumption, too.