Persist Makefile flags (2.6)

just fieldobj itself as sentinel of the fact a field object is used or
not, instead of using the filed length, that may be confusing both for
people and for the compiler emitting a warning.

1) Don't accept maxclients set to < 0
2) Allow maxclients < 1024, it is useful for testing.

lookupKeyByPattern() was implemented with a trick to speedup the lookup
process allocating two fake Redis obejcts on the stack. However now that
we propagate expires to the slave as DEL operations the lookup of the
key may result into a call to expireIfNeeded() having the stack
allocated object as argument, that may in turn use it to create the
protocol to send to the slave. But since this fake obejcts are
inherently read-only this is a problem.

As a side effect of this fix there are no longer size limits in the
pattern to be used with GET/BY option of SORT.

See https://github.com/antirez/redis/issues/460 for bug details.

This is not needed because every change in compiler/linker flags
triggers a cleanup.

This reflects that REDIS_*FLAGS will only be used for compilation of
Redis and not for its dependencies. Similarly, that FINAL_*FLAGS are
composed of other variables and holds the options that are finally
passed to the compiler and linker.

Lua global protection can now be simpified becuase we no longer have the
global() function. It's useless to occupy memory with this table, it is
also not faster because the metamethods we use are only called when a
global object does not exist or we are trying to create it from a
script.

After considering the interaction between ability to delcare globals in
scripts using the 'global' function, and the complexities related to
hanlding replication and AOF in a sane way with globals AND ability to
turn protection On and Off, we reconsidered the design. The new design
makes clear that there is only one good way to write Redis scripts, that
is not using globals. In the rare cases state must be retained across
calls a Redis key can be used.

I modified it for error in a previous commit doing search & replace.

Makefile now introduces Redis-specific CFLAGS / LDFLAGS. Gcov target fixed. Added comments to describe how it works.