fish: Move 'is_true' function to library utilities.

The 'is_true' function can be useful elsewhere, not just for parsing
guestfish command parameters.

This is just code motion.
environment: Use guestfs___is_true when parsing various boolean environment variables (RHBZ#1175196).

You can now use LIBGUESTFS_DEBUG=true (etc.)

You can disable debugging/tracing by setting LIBGUESTFS_DEBUG=0 (etc.)
fish: Add regression test for RHBZ#1175196.
v2v: Treat "oraclelinux" as if it was plain RHEL (RHBZ#1174280).