A few improvements to gen_mksurfdata_jobscript_single/multi

[ekluzek]

There are a few improvements to the gen_mksurfdata_jobscript_single/gen_mksurfdata_jobscript_mult scripts that would be helpful either for clarity, readability, or code cleanup.

This is in addition to #2439 and mostly are simple.

This mostly came up from review with @samrabin

https://github.com/ESCOMP/CTSM/pull/2427/files#r1543744893 https://github.com/ESCOMP/CTSM/pull/2427/files#r1543712951

• [x] To make it more clear that the output shell scripts are generated have them end with .sh (then it's more clear what's what)
• [x] Going with that change the name of the gen_mksurfdata_build.sh to gen_mksurfdata_build
• [ ] Improve names of some of the subroutines
• [ ] Improve names of the failure tests