master: fix typo from 2fdef85

2fdef85b6e167e9b875d6f9bb2147974f599071e introduced a typo. Fixing it.
Merge branch 'fix-2fdef85' into devel
Plugin.pm: enhance $statefile error handling
plugins: enhance spamstats and postfix_mailstats

Several enhancements :
- Port them to Plugin.pm
- Add state handling and logtailing
plugins: remove unneeded "use lib"

Commit 28a8944 added an uneeded "use lib".

It isn't needed as the Plugin.pm file shall be in the PERLLIB path.
check if $statefile exists but is not readable
fix error handling
fix "typo"

I'm an idiot
Merge branch 'devel' of github.com:ofosos/munin into devel