Minor Description
The recently added support (TOREE-437) for an alternate interrupt signal was implemented using the environment variable TOREE_ALTERNATE_SIGINT. However, we've recently learned that support for SPARK_YARN_USER_ENV is being removed in Spark 2.2 and its via that variable that TOREE_ALTERNATE_SIGINT is conveyed to the Toree kernel when launched as a Yarn cluster application (cluster mode).
As a result, and based on similar discussion on TOREE-443, it would be better to convey the need for an alternate interrupt signal via a command line option. This way, all launch modes of Toree could set this option in the same manner - via __TOREE_OPTS__ in kernel.json, for example.
The command-line option should be something like:
--alternate-sigint=<signal name>
with a description of something like:
Specifies the signal to use instead of SIGINT for interrupting a long-running cell. The value is a string and does not include the SIG prefix. Use of USR2 is recommended.
