[moz/cfg] Fix distcc/ccache/MAKE_JOBS

All three of these broke recently!

- MOZ_MAKE_FLAGS=-jfoo is stripped and replaced by the value of
  MOZ_PARALLEL_BUILD

- MOZ_MAKE_FLAGS="CC=foo" no longer works, but is no longer needed
  because:

- --with-compiler-wrapper added, replaces with-ccache

As an added bonus the old methods all turned into no-ops, include both
so we can still build older stuff with ccache/distcc
[moz/cfg] Add ff-dbg-minimal
[moz/cfg] Disable -O1 in ff-dbg-asan

Doesn't cause exhaustion in 64bit builds, and clang O1 still murders a
ton of debug info :(
[bin/mnuke] Actually nuke other build directories

the `cmd` was added as `showcmd` for debugging and never upgraded