ENH: Always execute global before filetype-specific.

Via new flag b:localrc_done. This allows filetype-specific settings to override global localrc settings.
ENH: Support compound filetype like "foo.bar".
ENH: Execute filetype-specific configuration after ftplugins.
Add version guard for older Vims.
Do not invoke after/ftplugin.vim stuff when localrc is disabled.
Also hook into file renamings to apply filetype-specific configuration.
Handle errors during sourcing of local configuration.
FIX: Prevent long network delays on Windows by proper path concatenation.

When a base path of "/" is concatenated with "/foo", the resulting "//foo" is interpreted by Windows as an UNC path, resulting in a slow network lookup. Fix this by properly combining paths instead of the simplistic concatenation.