Revitalise System.Posix.DynamicLinker.Module

[Bodigrim]

1. This module has virtually no documentation: there is neither a high-level overview of the API, nor any haddocks for individual functions.
2. withModule function is partial and will throw an error Prelude.tail: empty list when mdir is Just "". This must be dealt with one way or another.

unix/System/Posix/DynamicLinker/Module.hsc

Lines 96 to 106 in b085646

	withModule :: Maybe String
	-> String
	-> [RTLDFlags]
	-> (Module -> IO a)
	-> IO a
	withModule mdir file flags p = do
	let modPath = case mdir of
	Nothing -> file
	Just dir -> dir ++ if ((head (reverse dir)) == '/')
	then file
	else ('/':file)

[sjshuck]

Also true for System.Posix.DynamicLinker IMO. dlerror calls peekCString on the nullable result of dlerror(3p) and segfaults if there's no error. To keep backwards-compatible, it'd have to return "" instead, but down the line I wonder if a new major release co-existing API with things like dynamicLinkerError :: IO (Maybe String) isn't in order.