Implement OS-specific RPM repositories

The Frontier RPM repository is being broken up by OS version. There will be EL7, EL8, and EL9 repositories to support RPMs specific to those OS versions. In this first iteration, the EL7 and EL8 directories will be links since no RPMs specific to those versions are needed right now. The new directory design is backwardly compatible.

The ~dbfrontier/dist directory will contain these directories:

el7
el7-debug
el8
el8-debug
el9
el9-debug
rpms
rpms-debug

el7* and el8* will be links to the corresponding rpms or rpms-debug directories. el9 and el9-debug are actual directories, but their contents, except for the frontier-squid rpms, will be links to the corresponding files in rpms or rpms-debug.

rpm2web.sh is enhanced to release to the standard or EL9 directory, based upon whether it is being run on an EL9 system or not. It now contains a case statement that lists the packages that have OS-specific RPMs. Currently, this case statement only has frontier-squid. In future, if other packages will require OS-specific versions, they will have to be added to this case statement.

As a convenience, rpm2web.sh now accepts "-s source_directory" as an optional first parameter to specify the full path of the source directory from which to get the RPMs instead of the standard $HOME/rpms.

Shell commands that are to be run on the destination repository machine were removed from rpm2web.sh and put in the new file updaterepo.sh for ease of development and maintenance.

There is a companion merge request for small changes to the repo files in frontier-release: frontier-release!1 (merged)