remake should set REMAKEFLAGS (for -X and -x) and not MAKEFLAGS

From make-4.1/NEWS +102

====> Version 4.0 (09 Oct 2013)

• New variable: $(GNUMAKEFLAGS) will be parsed for make flags, just like
  MAKEFLAGS is. It can be set in the environment or the makefile, containing
  GNU make-specific flags to allow your makefile to be portable to other
  versions of make. Once this variable is parsed, GNU make will set it to the
  empty string so that flags will not be duplicated on recursion.

...

Behavior of MAKEFLAGS and MFLAGS is more rigorously defined. All simple
flags are grouped together in the first word of MAKEFLAGS. No options that
accept arguments appear in the first word. If no simple flags are present
MAKEFLAGS begins with a space. Flags with both short and long versions
always use the short versions in MAKEFLAGS. Flags are listed in
alphabetical order using ASCII ordering. MFLAGS never begins with "- ".

Given this, when -X or --debugger is parsed, that value should not be set in MAKEFLAGS or MFLAGS.

And as with issue #25, debugger commands, "step", "next", "finish" and "continue" would modify the REMAKEFLAGS environment variable.

No files or tests are named. Start by locating the existing handling of MAKEFLAGS and MFLAGS, then trace how -X, --debugger, and the debugger commands step, next, finish, and continue update environment flags. Done means those debugger-specific values are excluded from MAKEFLAGS and MFLAGS and are reflected in REMAKEFLAGS.