Adds -D option to pass home directory path on ADB command line

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This is a fix to add an option to allow passing of a path to
a home directory on the adb command line.

Usage:

 $ adb -D /path/to/my/home/dir ...

The home directory is selected in the following order:

 1) The -D option.
 2) The value of the 'HOME' environment variable.
 3) Built in defaults.

Files modified:
   adb.c
   adb.h
   adb_auth_host.c
   commandline.c
   usb_vendors.c