modes.h: init_gamma(): remove n3 and n4 arguments
added cipher feedback mode
gost28147.c: parse_args(): remove unnecessary blank line
gost28147.c: fix usage string
gost28147.c: main(): close source file before return

If mode is ecb and source file have incorrect size then close file
and exit.
gost28147.c: main(): close key file before return

If key file have incorrect size then close file and exit.
gost28147.c: main(): check existence of a key file
modes.h: cnt_crypt(): use compound assignment operators