Allow multiple icommands versions to be installed about irods_client_icommands

[mcv21]

On .deb-based systems, an approach based on update-alternatives would probably be the way to go

[mcast]

The need for this comes partly from our code, which needs to reach into different zones that may be on separate upgrade schedules.

An extra factor is the need for irods_environment.json > "irods_plugins_home" which we needed on old Ubuntu Precise with icommands in /opt/renci. This became incompatible at the user config file level with new Ubuntu Bionic icommands in /usr/bin.

As we have users who may run the commands from either OS with a shared homedir, and would prefer not to have another level of "which OS are we on? need to grab different config files!", it makes a window of time without a well-defined cutover date.

(Hoping this made sense - happy to clarify)

[kript]

Talking to other internal users, they would need to be able do an equivalent of ./configure --prefix=/opt && make && make install. If it were possible to build the iRODS client libraries and install them and the headers to /opt or /usr/local that should be enough.

This would be for building baton and samtools, as well as for building variable location icommands.