Avoid shadowing local variable names.

Avoid shadowing local variable names.

All of these changes avoid using the same name for two local variables
within a same function.  None of them are actual bugs as far as I can tell,
but any of them could be confusing to the casual reader.

The one in lib/ovsdb-idl.c is particularly brilliant: inner and outer
loops both using (different) variables named 'i'.

Found with GCC -Wshadow.