Add decrypt_files operation to the assuan engine
To be used to decrypt a list of filenames. * src/common.c (percent_escape): New. Taken from gpgEx to escape file names. * src/engine-assuan.c (op_assuan_start_decrypt_files): New. Handles the assuan transactions. * src/engine-assuan.h, src/util.h: Add prototypes * src/util.h (tohex_lower): New. Taken from gpgEx