main: add main_max and main_read_enum
main: move SIZE_MAX to main_params.size_max
main: add main_enum and main_enum_init
main.c: main_error: cosmetic fix
main: main_read_text: make it return status code
main: main_read_enum: make it fill main_enum and return a status code
main: add main_read_key_type
main: main_encrypt and main_decrypt: ask user for key type