Persuade tsearch/tsearch2 to work (or at least pass their regression

Persuade tsearch/tsearch2 to work (or at least pass their regression
tests) when using flex 2.5.31.  The fix is to *not* try to use palloc
and pfree for allocations within the lexer; when you do that, the
yy_buffer_stack gets freed at inopportune times.  The code is already
set up to do manual deallocation, so I see no particular advantage to
using palloc anyway.