Full mandoc locale support committed.

[Kristaps Dzonsons <kristaps@bsd.lv>]

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Hi,

With this last commit, initial [full] locale support has been fitted 
into mandoc!  Attached is eye-candy: a manual full of random Unicode 
input (\[uNNNN]) first with -Tascii, then with -Tlocale.

 From the manual:

    Locale Output
      Locale-depending output encoding is triggered with -Tlocale.
      This option is not available on all systems: systems without
      locale support, or those whose internal representation is not
      natively UCS-4, will fall back to -Tascii.  See ASCII Output
      for font style specification and available command-line
      arguments.

The check-ins:

   (1) http://mdocml.bsd.lv/archives/source/0920.html
   (2) http://mdocml.bsd.lv/archives/source/0919.html

This support is /very/ fast, and any overhead occurs if and only if 
-Tlocale is selected AND supported.  If this doesn't hold, then mandoc 
runs at -Tascii "native" speed.

The bad news: it comes at a price.  -Tlocale only works if Unicode 
code-point values (defined in the UCS-4 (-2?) standards) can be 
transformed directly into wide-character values usable by the system.  I 
check this with an optional C99 feature, __STDC_ISO_10646__.  See

  http://www.cl.cam.ac.uk/~mgk25/ucs/iso2022-wc.html

for details.  Unfortunately, this seems only to be exported on glibc. 
I'm told the conditions hold on OpenBSD and FreeBSD.  NetBSD?

If your system abides by these rules and doesn't export this symbol, 
please let me know and we can special-case the macro test for this 
feature.  There is a way to convert from Unicode to a system's 
wide-character support without this feature, but it isn't pretty.  I'll 
probably have to implement this anyway for portability.  For now, if a 
system doesn't do __STDC_ISO_10646__, -Tlocale is a synonym for -Tascii.

Note that if this method is unilaterally hated, it's easy to switch to 
another method.  This was simply the fastest, simplest, and most 
transparent to implement.  All of the logic either way is in one file, 
and easy to manipulate:

  http://mdocml.bsd.lv/cgi-bin/cvsweb/term_ascii.c?cvsroot=mdocml

Thoughts?

Kristaps

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