manpage: lua: fixes and improvements

Some things were outdated, or outright wrong.
manpage: fix typo

Fixes #1384.
Add Javascript scripting support using MuJS

Almost identical API to the Lua support, except:
- The global mp, mp.msg and mp.utils are always available
- On errors returning undefined and the error is at mp.last_error_string
- Timers are JS "standard" global set/clear Timeout/Interval/Immediate
- Supports CommonJS style module/require
- At mp.utils: read_file (any), and load_file, run_file for js files
- Global print and dump functions
- mp.options currently unsupported

Required Mujs (clone, make, make install):
- http://mujs.com/ - git://git.ghostscript.com/mujs.git
- Mirror - https://github.com/ccxvii/mujs
JS: 'reqire' was broken for scripts
JS: minor cleanups

- Stray printf's
- Remove names from anonymous functions which are unlikely
  to show on stack traces to reduce clutter
javascript: C style cleanups
options: allows javascript also when Lua is not configured
js: replace lua-opts for script-opts
mu2duk-WIP
mu2duk - seems fully working after few short tests
mu2duk: comments and cleanups. TODO: support either via config
mu2duk: cleanups and refactoring
mu2duk: MUD_WRAPPER now use nargs at runtime
mu2duk: provide js backend info
mu2duk: fix js_Setlength, simplify error print
mu2duk: clearer wrapper with JS_C_FUNC
pr1351: simplified file functions
pr1351: cleanups (except for unused/legacy stuff)
js: simplification of object properties enumeration
js: allow fast switching between backends during development