Move file locking into the RTS, fixing #629, #1109

File locking (of the Haskell 98 variety) was previously done using a
static table with linear search, which had two problems: the array had
a fixed size and was sometimes too small (#1109), and performance of
lockFile/unlockFile was suboptimal due to the linear search.
Also the algorithm failed to count readers as required by Haskell 98
(#629).

Now it's done using a hash table (provided by the RTS).  Furthermore I
avoided the extra fstat() for every open file by passing the dev_t and
ino_t into lockFile.  This and the improvements to the locking
algorithm result in a healthy 20% or so performance increase for
opening/closing files (see openFile008 test).

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