MINOR: cfgparse: expose the keyword modifier to the keyword parsers

MINOR: cfgparse: expose the keyword modifier to the keyword parsers

Section parsers receive the keyword modifier ("no" or "default") found in
front of the keyword as their last argument, but the keyword parsers which
are registered in a cfg_kw_list only receive the arguments, the file name
and the line number. As a consequence, a keyword supporting negation such
as "busy-polling" cannot be moved out of a section parser.

Let's just expose the modifier of the line being parsed in a new global
variable, cfg_curr_kwm, which is set for each new line in readcfgfile()
and reset once the file is completely parsed. This way the few keyword
parsers which are interested in it may consult it, the same way they
already consult "file" and "line".