Commit f2771aa614 introduced a bug where some switches were left out
of MAKEFLAGS. Instead of resetting switches, get the same results by
filtering out duplicates.
* src/makeint.h: Remove reset_switches.
* src/main.c: (reset_switches): Remove reset_switches.
* (main): Remove call to reset_switches.
* (decode_switches): Filter out duplicate flags.
* src/variable.c: (set_special_var): Remove call to reset_switches.
* tests/scripts/variables/MAKEFLAGS: Verify that duplicate flags are
properly filtered out.