${WORLDTMP}/legacy/usr/libexec will only have libexec/ bits that we've pushed as bootstrap tools, so this is generally safe to include prior to PATH. The following are the ramifications of this change:
- BPATH addition gets us at least bootstrap flua in WMAKEENV path for buildenv, for those earlier systems where it's bootstrapped still (< CURRENT)
- TMPPATH addition gets the host flua path into WMAKEENV for !BUILD_WITH_STRICT_TMPPATH (the default) buildenv
- Reworked the sysent target to just set PATH and let it get worked out in src.lua.mk or individual sysent makefiles -- this gives us back the ability to overwrite LUA_CMD and use a different/external lua for these targets