Warnings with clang when using long double for Lua floats.

The allocation of a userdata for the state of the warn system can
cause a panic if it fails; 'luaL_ref' also can fail. This commit
re-implements the warn system so that it does not need an explicit
state. Instead, the system uses different functions to represent
the different states.

Comments in makefile and function 'l_str2d'.

Several of its callers needed that case and had to do the check
themselves.

The function 'lua_pushfstring' is seldom called with large strings,
there is no need to optimize too much for that cases.

With MMBIN instructions, there are fewer opcodes that need to update
'L->top', so that change does not seem to pay for the increased
complexity.

Instead of updating 'L->top' in every place that may call a
metamethod, the metamethod functions themselves (luaT_trybinTM and
luaT_callorderTM) correct the top. (When calling metamethods from
the C API, however, the callers must preserve 'L->top'.)

OP_NEWTABLE is followed by an OP_EXTRAARG, so that it can keep
the exact size of the array part of the table to be created.
(Functions 'luaO_int2fb'/'luaO_fb2int' were removed.)

Several small changes from feedback on 5.4 alhpa rc1 (warnings,
typos in the manual, and the like)

- new error message for "attempt to assign to const variable"
- note in the manual about compatibility options
- comments
- small changes in 'read_line' and 'pushstr'