plugin/diskstats: fix for negative stats, relax warning limits
plugins: fix some external binary paths

These work for RHEL5/6 and Fedora. What ever your system is, /usr/local/
should not be the default.
plugins/http_loadtime: more wget/time fixes

In autoconf $wget_bin is undefined, use wget as elsewhere. Also direct
stdout to null to make it work.

When running with bash "time" is internal bash command and it's output
can't be redirected. Force plugin to use external /usr/bin/time command
so that redirect works.
plugins/lpstat: fix autodetection

"lpstat -r" outputs data even if no printers are configured, resulting
lpstat plugin to print errors when run. Use "lpstat -v" to detect
if there really are some printers.
plugins/snmp__if_multi: prefer ifAlias for alias name

Also fix if snmpinfoX/ifHCInOctets is undefined.
plugins/postfix_mailstats: support proxy-rejected (amavis) log lines
asyncd: add more signal handlers.

HUP handler is needed for "ssh -t node service munin-asyncd restart" to work.
Without it only "ssh node" works, but not with -t.
INT/TERM handlers are needed to kill asyncd nicely in init.d scripts.