core,w32: Pass file names with forward slashes to gpg and gpgtar

src/conversion.c, src/util.h (_gpgme_replace_backslashes): New.
src/engine-gpg.c (_add_arg): Add argument file_name. On Windows, replace
backslashes in file name arguments with forward slashes.
(add_arg_ext, add_arg_with_locp, add_arg, add_arg_pfx, add_gpg_arg,
add_gpg_arg_with_value, add_arg_len): Adjust call of _add_arg.
(add_file_name_arg, add_file_name_arg_len, add_gpg_arg_with_file_name):
New.
(add_file_name_arg_or_data): Call add_file_name_arg for file name.
(gpg_new): Call add_gpg_arg_with_file_name for home directory.
(gpg_decrypt): Call add_file_name_arg for output directory and output
file name.
(append_args_from_recipients_string): Call add_file_name_arg_len for
recipients file name.
(gpg_encrypt): Call add_file_name_arg for output file name, base
directory and input file name. Call add_gpg_arg_with_file_name to set
file name stored in message.
(gpg_encrypt_sign): Call add_file_name_arg for output file name, base
directory and input file name. Call add_gpg_arg_with_file_name to set
file name stored in message.
(gpg_sign): Call add_file_name_arg for output file name, base directory
and input file name. Call add_gpg_arg_with_file_name to set file name
stored in message.
(gpg_verify): Call add_file_name_arg for output directory and output

file name.

By passing all file names with forward slashes to gpg and gpgtar we
avoid problems caused by the quoting of backslashes.

• GnuPG-bug-id: T7141