Accept a token number in a %left, %right, or %nonassoc for POSIX

Accept a token number in a %left, %right, or %nonassoc for POSIX
conformance.  Reported by Tim Josling at
<http://lists.gnu.org/archive/html/bug-bison/2008-01/msg00010.html>.
* NEWS (2.3a+): Mention.
* doc/bison.texinfo (Precedence Decl): Describe how literal strings
and code numbers are treated by precedence declarations.
* src/parse-gram.y (precedence_declaration): Use symbols.prec instead
of symbols.1.
(symbols.prec): New, just like symbols.1 but uses symbol.prec instead
of symbol.
(symbol.prec): New, just like symbol but allows INT.
* src/symtab.c (symbol_user_token_number_set): Remove an aver that no
longer holds.
* tests/regression.at (Token number in precedence declaration): New
test case.