Refactor the fallocate call.
Explicitly test for WT_FALLOCATE_NOT_AVAILABLE, that way code doesn't
assume a value of 0 means something special and we can use an enum for
fallocate_available.
Don't lose real errors (we ignore the error the first time the stub
functions are called, so we can return the "real" error from them).
Fix comparison.
Merge pull request #1387 from wiredtiger/fallocate-refactor

Refactor the fallocate call.