fs_mgr: remount: add -v option

By default print only ERROR message, or messages without a source
(eg: [fs_mgr]) library.  Print all messages to stderr if the -v
option is added.  Always log all messages to the Android logger.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Test: adb-remount-test.sh
Change-Id: I131315d4bae6ed29991043033811cf22e6032f8b