[FIXED JENKINS-25533] Favor the new style of form submission over the legacy form

[FIXES JENKINS-24272] jnlp slaves fail to reconnect when master restart

During master restart only attempt to reconnect the slave after the master has
finished restarting.

- There is still the issue with adding isConnecting() polluting the NodeProvisioner, but this is
  a simple, low risk but important change

Fix Swedish spelling of 'tog'

Include parameter name in exception when value is invalid

Some null annotations and javadoc fix

Fixing 9162c3cb

Not fully solved, since the scannedDirs field can still grow to be large, but at least clearing files/dirsNotIncluded.
Also imposing a 5s timeout on the scan regardless of file count, and defining a user-customizable bound.