Set FD_CLOEXEC after each file open

Summary: as subject. This is causing problem in adsconv. Ideally, this flags should be set in open. But that is only supported in Linux kernel ≥2.6.23 and glibc ≥2.7.

Test Plan:
db_test

run db_test

Reviewers: dhruba, MarkCallaghan, haobo

Reviewed By: dhruba

CC: leveldb, chip

Differential Revision: https://reviews.facebook.net/D10089

include/leveldb/env.h

@@ -385,6 +385,9 @@ class EnvOptions {
 
    // If true, then use mmap to write data
    virtual bool UseMmapWrites() const = 0;
+
+   // If true, set the FD_CLOEXEC on open fd.
+   virtual bool IsFDCloseOnExec() const = 0;
 };
 
 // Log the specified data to *info_log if info_log is non-nullptr.

include/leveldb/options.h

@@ -415,6 +415,9 @@ struct Options {
 
   // Allow the OS to mmap file for writing. Default: true
   bool allow_mmap_writes;
+
+  // Disable child process inherit open files. Default: true
+  bool is_fd_close_on_exec;
 };
 
 // Options that control read operations