ipfilter: simplify ipf_proxy_check() return codes

ipf_proxy_check() returns -1 for an error and 0 or 1 for success.
ipf_proxy_check()'s callers check for error and if the return code
is 0, they change it to 1 prior to returning to their callers. Simply
by returning -1 or 1 we reduce complexity and cycles burned changing
0 to 1.

MFC after: 1 week