Left4Dead2 script made safer for dirnames/filenames with spaces, and some other stuff

* Made the script safer with destinations with spaces in them by
    double quoting some of the variables that pertain to dirnames
* Cleaned up a couple of lines with trailing spaces
* Added a function for scriplogging, and replaced the lines where it
    uses them. This has been tested, it works just like it used to
* Had to replace install.txt steam script with a commandline (like for
    update), because steam install script treats quotes as literals:
    http://forums.steampowered.com/forums/showthread.php?t=2896163
    The "fix" in this forum post is not using spaces :)
* Replaced some "mkdir -v" commands with "mkdir -pv" so as to avoid
    getting errors if the directory already exists

Tested on a completely fresh homedir:
- install
- start
- stop
- restart
- update
- monitor
- email-test
- details
- backup
- console
- debug

All test completed successfully!