frameencoder: nit
sei: emit SEI describing the encoder and encode options

example:
% x265 ../sintel_trailer_2k_480p24.y4m o.bin
% strings o.bin | head
x265 (build 26) - 1.2+74-a5024bfc0b50:[Mac OS X][clang 5.1.0][64 bit] 8bpp -
H.265/HEVC codec - Copyright 2013-2014 (c) Multicoreware Inc - http://x265.org
- options: 856x480 fps=24/1 bitdepth=8 wpp ctu=64 tu-intra-depth=1
tu-inter-depth=1 me=1 subme=2 merange=57 no-rect no-amp max-merge=2
no-early-skip no-fast-cbf rdpenalty=0 no-tskip no-tskip-fast
strong-intra-smoothing no-lossless no-cu-lossless no-constrained-intra open-gop
interlace=0 keyint=250 min-keyint=24 scenecut=40 rc-lookahead=20 bframes=4
bframe-bias=0 b-adapt=2 ref=3 weightp no-weightb aq-mode=2 aq-strength=1.00
cbqpoffs=0 crqpoffs=0 rd=3 signhide lft sao sao-lcu-bounds=0 sao-lcu-opt=1
b-pyramid cutree rc=crf crf=28.0 qcomp=0.60 qpmin=0 qpmax=51 qpstep=4
ipratio=1.40 pbratio=1.30
encoder: remove unused variables
encoder: nits