up2date-2.9.3-1.2.1AS and rhn_register-2.9.3-1.2.1AS barf if proxy
user or proxy password in the config file are entirely numeric.

I suppose we could use the default or previous value to tell whether
the value is to be converted to int or remain as a string.  E.g., if
the key is already defined, with a value that is an integer, we
convert the new value to integer.  If it is already defined to a
string, we leave the new value alone.  Otherwise, we may apply the
current heuristics, just to make sure we're not unexpectedly changing
behavior.