util: New ovs_retval_to_string() function.

util: New ovs_retval_to_string() function.

Many OVS functions return 0, EOF, or errno. There are several places in the
codebase where a return value is converted to a string. All must decide whether
the return value is set, and if it is, whether it is an errno value, EOF, or
otherwise invalid. This commit consolidates that code.

Reviewed by Ben Pfaff.