Fix PR gdb/21954: make 'unset environment' work again
    
    When I made commit 9a6c7d9c021cfeb290d76584db7a01e57e7c3d4e, which
    C++-fied gdb/common/environ.[ch], I mistakenly altered the behaviour
    of the 'unset environment' command.  This command, which should delete
    all environment variables, is now resetting the list of variables to
    the state they were when GDB was started.
    
    This commit fixes this regression, and also adds a test on
    gdb.base/environ.exp which really checks if 'unset environment'
    worked.
    
    gdb/ChangeLog:
    2017-08-15  Sergio Durigan Junior  <sergiodj@redhat.com>
    
    	PR gdb/21954
    	* infcmd.c (unset_environment_command): Use the 'clear' method on
    	the environment instead of resetting it.
    
    gdb/testsuite/ChangeLog:
    2017-08-15  Sergio Durigan Junior  <sergiodj@redhat.com>
    
    	PR gdb/21954
    	* gdb.base/environ.exp: Add test to check if 'unset environment'
    	works.