Allow user to specify profile sort column on the command line. (#4056)

- Add -P <STAT> argument so that caller can specify a sort column for
  cProfile. Can specify multiple columns with commas. e.g.:
      spack -P cumtime,module

- Add --lines option to Spack spec to control number of profile lines
  displayed

- Sort by time by default (because it works in all Python versions)

- Show sort column options in command help.

- Do a short profile run in the unit tests.