Bug 1060: Use libtre for regexp searches.

Bug 1060: Use libtre for regexp searches.

When the user tells ELinks to search for a regexp, ELinks 0.11.0
passes the regexp to regcomp() and the formatted document to
regexec(), both in the terminal charset.  This works OK for unibyte
ASCII-compatible charsets because the regexp metacharacters are all in
the ASCII range.  And ELinks 0.11.0 doesn't support multibyte or
ASCII-incompatible (e.g. EBCDIC) charsets in terminals, so it is no
big deal if regexp searches fail in such locales.

ELinks 0.12pre1 attempts to support UTF-8 as the terminal charset if
CONFIG_UTF8 is defined.  Then, struct search contains unicode_val_T c
rather than unsigned char c, and get_srch() and add_srch_chr()
together save UTF-32 values there if the terminal charset is UTF-8.
In plain-text searches, is_in_range_plain() compares those values
directly if the search is case sensitive, or folds them to lower case
if the search is case insensitive: with towlower() if the terminal
charset is UTF-8, or with tolower() otherwise.  In regexp searches
however, get_search_region_from_search_nodes() still truncates all
values to 8 bits in order to generate the string that
search_for_pattern() then passes to regexec().  In UTF-8 locales,
regexec() expects this string to be in UTF-8 and can't make sense of
the truncated characters.  There is also a possible conflict in
regcomp() if the locale is UTF-8 but the terminal charset is not, or
vice versa.

Rejected ways of fixing the charset mismatches:

* When the terminal charset is UTF-8, recode the formatted document
  from UTF-32 to UTF-8 for regexp searching.  This would work if the
  terminal and the locale both use UTF-8, or if both use unibyte
  ASCII-compatible charsets, but not if only one of them uses UTF-8.

* Convert both the regexp and the formatted document to the charset of
  the locale, as that is what regcomp() and regexec() expect.  ELinks
  would have to somehow keep track of which bytes in the converted
  string correspond to which characters in the document; not entirely
  trivial because convert_string() can replace a single unconvertible
  character with a string of ASCII characters.  If ELinks were
  eventually changed to use iconv() for unrecognized charsets, such
  tracking would become even harder.

* Temporarily switch to a locale that uses the charset of the
  terminal.  Unfortunately, it seems there is no portable way to
  construct a name for such a locale.  It is also possible that no
  suitable locale is available; especially on Windows, whose C library
  defines MB_LEN_MAX as 2 and thus cannot support UTF-8 locales.

Instead, this commit makes ELinks do the regexp matching with regwcomp
and regwexec from the TRE library.  This way, ELinks can losslessly
recode both the pattern and the document to Unicode and rely on the
regexp code in TRE decoding them properly, regardless of locale.

There are some possible problems though:

1. ELinks stores strings as UTF-32 in arrays of unicode_val_T, but TRE
   uses wchar_t instead.  If wchar_t is UTF-16, as it is on Microsoft
   Windows, then TRE will misdecode the strings.  It wouldn't be too
   hard to make ELinks convert to UTF-16 in this case, but (a) TRE
   doesn't currently support UTF-16 either, and it seems possible that
   wchar_t-independent UTF-32 interfaces will be added to TRE; and (b)
   there seems to be little interest on using ELinks on Windows anyway.

2. The Citrus Project apparently wanted BSD to use a locale-dependent
   wchar_t: e.g. UTF-32 in some locales and an ISO 2022 derivative in
   others.  Regexp searches in ELinks now do not support the latter.

[ Adapted to elinks-0.12 from bug 1060 attachment 506.
  Commit message by me.  --KON ]