Add new flags for THMapAllocator

[apaszke]

• TH_ALLOCATOR_MAPPED_FROMFD uses an existing file descriptor for
  mapping (and steals it)
• TH_ALLOCATOR_MAPPED_KEEPFD doesn't close the file descriptor
  until the mapping is freed
• TH_ALLOCATOR_MAPPED_UNLINK unlinks the file immediately after
  mapping it to memory

Also, now it's using fstat to check the file size (instead of lseek, which alters the fd state).

[soumith]

cc: @akfidjeland @timharley wrt the change from lseek to fstat. It's harmless, but want to keep you in the loop.

[timharley]

Thanks @soumith

[soumith]

Thanks @apaszke !

[tworec]

I have problem with THAllocator.c line 262

In [1]:
s=torch.FloatStorage('/tmp/2rec', true, 10000)

gives me

unable to stretch file </tmp/2rec> to the right size at /home/tworec/torch/pkg/torch/lib/TH/THAllocator.c:262
stack traceback:
    [C]: at 0x7fd308444330
    [C]: in function 'FloatStorage'
    [string "s=torch.FloatStorage('/tmp/2rec', true, 10000..."]:1: in main chunk
    [C]: in function 'xpcall'
    /home/tworec/torch/install/share/lua/5.1/itorch/main.lua:210: in function </home/tworec/torch/install/share/lua/5.1/itorch/main.lua:174>
    /home/tworec/torch/install/share/lua/5.1/lzmq/poller.lua:75: in function 'poll'
    /home/tworec/torch/install/share/lua/5.1/lzmq/impl/loop.lua:307: in function 'poll'
    /home/tworec/torch/install/share/lua/5.1/lzmq/impl/loop.lua:325: in function 'sleep_ex'
    /home/tworec/torch/install/share/lua/5.1/lzmq/impl/loop.lua:370: in function 'start'
    /home/tworec/torch/install/share/lua/5.1/itorch/main.lua:389: in main chunk
    [C]: in function 'require'
    (command line):1: in main chunk
    [C]: at 0x00406670

[soumith]

@tworec this was fixed 13 days ago via #817

[tworec]

yes, indeed