thead safety for VSIMemFileSysHandler

[list672000@…]

The Intel Thread Checker is reporting "read/write data race" errors when run 
with a program that has multiple threads which call VSIFileFromMemBuffer(..), 
GDALOpen, VSIUnlink, etc. (all for reading rasters).

Many of the errors seem to be related to the accessing of the 
VSIMemFileSystemHandler's oFileList data member.

[warmerdam]

I have applied a patch to cpl_vsi_mem.cpp to protect access to oFileList
with a mutex.  I also added some notes on multithreading issues at
the top of the file (and in cpl_vsil.cpp).