add freebsd build glue
use sysctl over /proc to get exe path on freebsd
consistently initialize adb_mutex_t

  usb_libusb.c:40:20: error: static declaration of 'usb_lock' follows non-static
	declaration
  static adb_mutex_t usb_lock = ADB_MUTEX_INITIALIZER;
		     ^
  ./mutex_list.h:15:11: note: previous definition is here
  ADB_MUTEX(usb_lock)
	    ^
  ./sysdeps.h:318:45: note: expanded from macro 'ADB_MUTEX'
  #define  ADB_MUTEX(x)   extern adb_mutex_t  x;
					      ^
merge adb upstream platform/system/core.git@928116c

fastboot requires more work e.g., ext4_utils and libsparse
enable terminal handling based on autoconf
enable fork/exec based on autoconf
enable symlinks based on autoconf