Make our popt exec path more useful, permit environment override

Make our popt exec path more useful, permit environment override
- In 4.4.x days, we used to have bunch of helper binaries in /usr/lib/rpm
  executed through popt ping-pong, but there's nothing now that we'd
  want to execute through aliases. Instead use bindir for the
  default path, and additionally permit overriding through
  RPM_POPTEXEC_PATH environment variable (mostly probably useful for
  the testsuite)