write: Get rid of gengetopt's string parser.

It causes gengetopt to generate quite some additional code for
the string parsers of all writers. Moreover, this string parser is
inferior to create_argv() and create_shifted_argv() of string.c as
gengetopt's parser does not honor any quoting at all.

This commit changes the signature of the ->parse_config_or_die method
of struct writer to take an (argc, argv) pair instead of a string.  All
writers can thus call the vanilla command line parser of gengetopt. The
single user in write_common.c now calls ->parse_config_or_die()
with an (argc, argv) pair obtained from create_shifted_argv().