Sentinel: Improve INFO command behavior

Improvements:
  - Return empty string if asking for non-existing section (INFO foo)
  - Fix potential memory leak (caused by sdsempty() then returned if >2 args)
  - Clean up argument parsing
  - Allow "all" as valid section (same as "default" or zero args currently)
  - Move strcasecmp to end of evaluation chain in conditionals

Also, since we're C99, I moved some variable declarations to be closer
to where they are actually used (saves us from needing to free an empty info
if detect argument errors up front).

Closes #1915
Closes #1956