plugins/Plugin.pm: fix bad state file handling (fixed)

This commit is a redo of bogus bf7a258081d64bb6174f83482d5d6dd53cb5909a
and it includes the fix from sweetpea in ticket 1342.

Closes: #1342
Merge branch 'fix-state' into stable-2.0
Fix for munin-cgi-graph crash in trend and predict.

 - use get_field_name on the fieldnames for trend and predict.  A too long
   internal field name would make munin-(cgi-)graph crash.

 - bonus fix: use the label, and not the internal name, for the predict and
   trend labels.  (df graph will use "/home predict" instead of "_dev_vg0_home
   predict", or similar)
master: adding ":" to the allowed chars in CGI

":" is a valid char for an IPv6-enabled "ip_" plugin
Merge branch 'fix-ipv6' into stable-2.0
Merge branch 'trend_predict_crash' into stable-2.0
dev_scripts: automate async install

Also installs munin-async(d), so debugging it is eased.
master: re-enable debugging output in GraphOld.pm

Commit e0a1fe504d93021d9e2f2b8c2fc6eb2234474a64 added support for the --debug
so it could be launch with it.

This also adds the --debug behavior. It still doesn't log anything in normal
mode
Merge branch 'fix-debug-graph' into stable-2.0
asyncd: use the same rules as munin-update

munin-update does some squashing on invalid chars when using non-multigraph
plugins. This is done transparently when using the plain old TCP transport.

As munin-async(d) uses the spoolfetch transport that is multigraph-based, it
has the side effect not squashing the root plugin name. This patch squashes the
root plugin name in the same manner as done by munin-update.

It therefore enables to insert the munin-async(d) couple in-place to benefit
transparently.

Closes D:710529
ChangeLog for  2.0.15