Remove temporary test file
Improve handling of stores and streams

- Substitute a cat /store/ command with the corresponding sgsh-tee
  version.
- Allow stores to be read in the scatter block.
- Refactor the stream and store substitution code into a separate function
  so that it can be called from both the scatter and gather blocks.
- Close command groups in a separate line to make it possible
  to use HERE documents within the scatter block.

These measures can be used to avoid the deadlock caused by the current
version of example/web-log-report.sh on very large logs.
Avoid deadlock caused by successive cat commands

Each of the cat commands depends on its preceding one. Therefore,
the corresponding stream file is not opened, until the preceding
one finishes. This prevents the shell from opening the output
redirection to the corresponding write end of the named pipe,
and thereby never launching the corresponding command.

Solve the problem by using a single cat command, which the
d50f56d34 version of sgsh replaces with a call to sgsh-tee,
which opens all the input files upon startup.

TODO: Have sgsh automatically flag such deadlocks.
Have sgsh-test test e.g. scripts for syntax errors

Change sgsh -n to actually test for syntax errors.