Generate relocatable loadLSST.* scripts

The purpose of this proposed modification is to make `newinstall.sh` generate relocatable `loadLSST.*sh` scripts. This is a necessary (but not sufficient) ingredient for making the LSST software distribution relocatable.

If not already set, all the generated `loadLSST.*sh` scripts set at execution time the `LSST_HOME` variable to the directory where those shell-specific scripts are located. The current version of those scripts set that value to the directory where `newinstall.sh` was downloaded. The other components to bootstrap the installation (i.e. EUPS, Anaconda, git, etc.) are set up from the value of the `LSST_HOME`.

Please note that as of EUPS v1.3.0 the shell KSH is not supported since there is no KSH-specific `setups.ksh` for EUPS [see issue [DM-380](https://jira.lsstcorp.org/browse/DM-380)]. The `loadLSST.ksh` generated script takes that into account and terminates gracefully.
I'd rename this to generate_loader_bash(), as you use BASH_SOURCE in the implementation.