Finally fix the `install_server.sh` script.

I finally found the time to fix most of the outstanding issues regarding the `install_server.sh` script.

I used [bats](https://github.com/sstephenson/bats) and Vagrant to quickly test atleast some aspects of the script (on ubuntu, debian and centos). My test files can be found here: [gist](https://gist.github.com/badboy/6f81b2e0035c6dc8c110)
The `install_server.sh` script should now be as POSIX-compatible as I know.

The following pull requests were used as reference and thus thanks to all contributors:

* https://github.com/antirez/redis/pull/960, thanks @chilts (install script name)
* https://github.com/antirez/redis/pull/909, thanks @jimothyGator (missing space)
* https://github.com/antirez/redis/pull/781, thanks @calebperkins (`id -u`)
* https://github.com/antirez/redis/pull/724, thanks @quiver (missing space)
* https://github.com/antirez/redis/pull/1498, thanks @jw2013 (missing spaces)
* https://github.com/antirez/redis/pull/1486, thanks @alonisser (correct paths when not inside utils dir, fixes #1485 #1486)
* https://github.com/antirez/redis/pull/1345, thanks @GoldenWings91 (missing space)
* https://github.com/antirez/redis/pull/1148, thanks @nsabine (missing spaces)
* https://github.com/antirez/redis/pull/1146, thanks @TrafeX (removed relative paths, enabled backslash escapes in echo)
* https://github.com/antirez/redis/pull/1038, thanks @jumping (added double quotation marks)
* https://github.com/antirez/redis/pull/1066, thanks @melvyn-sopacua (always include sysv init info and avoid ugly "echo -e")

All of the following issues can be closed as well, they reported bugs the above pull requests solved:

* https://github.com/antirez/redis/issues/864
* https://github.com/antirez/redis/issues/840
* https://github.com/antirez/redis/issues/816
* https://github.com/antirez/redis/issues/505
* https://github.com/antirez/redis/issues/363
* https://github.com/antirez/redis/issues/259
* https://github.com/antirez/redis/issues/1539
* https://github.com/antirez/redis/issues/1485
* https://github.com/antirez/redis/issues/1399
* https://github.com/antirez/redis/issues/1228
* https://github.com/antirez/redis/issues/1099
* https://github.com/antirez/redis/pull/1037


There is one pull request left: https://github.com/antirez/redis/pull/1224
It mostly changes the init script a little bit, I'm not sure if this is really needed here.