script: add the -o/--output-limit option. Fix race test.

script: add the -o/--output-limit option. Fix race test.

When script is used on a host with a relatively small free disk space, it
is sometimes desirable to limit the size of the captured output. This
can now be enforced with the --output-limit option.

The --output-limit option lets the user specify a maximum size. The program
uses the size parsing from strutils and thus supports the usual
multiplicative suffixes (kiB, KB, MiB, MB, etc.). After the specified
number of bytes have been written to the output file, the script program
will terminate the child process.

Due to buffering, the size of the output file might exceed the specified
limit. This limit also does not include the start and done messages.

The race test was throwing an error dur to a variable being "" in some cases.
Quoting the variable in the equal test took care of that test.

[kzak@redhat.com: - use done() to stop script
                  - count also timing file
                  - remove unnamed member initialization in ctl struct
                  - add to bash-completion]

Signed-off-by: Fred Mora <fmora@datto.com>
Signed-off-by: Karel Zak <kzak@redhat.com>