adbd: Don't close/reopen FFS ep0 on disconnect

In case of bulk read/write failure or disconnect, transport_usb
calls usb_ffs_kick in order to force the daemon to re-open the
FunctionFS ep files as a way to soft restart the kernel driver.

However, there's no reason to always close the ep0 file, as it
would need to be reopened and the descriptors rewritten--this
is unnecessary, especially in the simple/frequent case of cable
disconnect, and causes the kernel driver to unbind and bind *all*
of the function drivers. This is causing USB Chapter 9 tests to fail.

Thus, try to reuse the same ep0 file handle across reconnects. In
case there is a failure, the ep1/ep2 opens would fail and all the
file handles get dropped. For the adb root case, the entire daemon
gets restarted anyway.

CRs-fixed: 571846
Change-Id: I0840085c52a0795dcb2d751c39aa4a436c039ee2
Fix a bunch of small system/core bugs.

Missing frees in:
  adb/file_sync_client.c
  fastboot/fastboot.c
  libsparse/output_file.c

Missing closedirs in:
  adb/file_sync_service.c
  cpio/mkbootfs.c
  libcutils/dir_hash.c

Potential buffer overrun in:
  gpttool/gpttool.c

Incorrect NULL check in:
  libsparse/backed_block.c

Bug: https://code.google.com/p/android/issues/detail?id=61564
Change-Id: If97838a9e73a77aef7f416c31c237ce1fca4ce21