Renamed 'empty_string' to 'empty_pike_string' to remove name clash with PHP.

Also made Parser.HTML and spider modules use  'empty_pike_string' rather than
their own.

Rev: src/cpp.c:1.103
Rev: src/modules/Parser/html.c:1.153
Rev: src/modules/spider/spider.c:1.116
Rev: src/stralloc.c:1.144
Rev: src/stralloc.h:1.68