This commit changes what provided by PR #3315 (merged) in order to
let the user specify the log level as a string.

The define could be also used, but when this happens, they must be
decoupled from the defines in the Redis core, like in the other part of
the Redis modules implementations, so that a switch statement (or a
function) remaps between the two, otherwise we are no longer free to
change the internal Redis defines.

module.

In modules we fill a set of function pointers defined in redismodule.h,
populating a set of APIs that are callable from the module. We use this
manual process instead of resorting to dynamic linking so that we have
exact control on how we pass the API to the module, and we can even pass
different functions for the same name, depending on the API version
declared by the module.

However if the function pointers in redismodule.h and the functions
defined in module.c have the same name, they conflict since the core
exports the symbols to the module.

There is probably some compiler flags trick to avoid this, but in order
to be safer in the future and be more easily compatible with different
builidng systems, this commit changes the internal function prefix from
RedisModule_ to RM_, so for example:

    RM_StringSet() will be exported as RedisModule_StringSet()