(decode_switches): Loop until optind hits ARGC, not just until getopt_long

(decode_switches): Loop until optind hits ARGC, not just until getopt_long
returns EOF.  Initialize C to zero before loop; in loop if C is EOF, set
optarg from ARGV[optind++], else call getopt_long.
(decode_env_switches): Use variable_expand instead of
allocated_variable_expand.  Allocate a fresh buffer to copy split words
into; scan characters by hand to break words and debackslashify.
(shell_quote): New function.
(define_makeflags): Allocate doubled space for switch args, and command
variable names and values; use shell_quote to quote those things.