win32: add mmap() emulation

Makes all of overlay_add work on windows/mingw.

Since we now don't explicitly check for mmap() anymore (it's always
present), this also requires us to make af_export.c compile, but I
haven't tested it.
command: overlay_add: more flexible treatment of offset parameter

Essentially, don't make it the mmap() argument, and just add it to the
memory address. This hides tricky things like alignment reequirements
from the user.

Strictly speaking, this is not entirely backwards compatible: this adds
the regression that you can't access past 2 or 4 GB of a file on 32 bit
systems anymore. But I doubt anyone cared about this.

In theory, we could be clever, and just align the offset manually and
pass that to mmap(). This would also be transparent to the user, but
minimally more effort, so this is left as exercise to the reader.
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
pr1351: simplified file functions
pr1351: cleanups (except for unused/legacy stuff)
pr1351: remove undocumented API which the OSC uses
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
mu2duk: simplification of object properties enumeration