* guix/scripts/lint.scm (report-tabulations, report-trailing-white-space,
  report-long-line, report-formatting-issues, check-formatting): New
  procedures.
  (%formatting-reporters): New variable.
  (%checkers): Add 'formatting' checker.
* tests/lint.scm ("formatting: tabulation", "formatting: trailing white
  space", "formatting: long line", "formatting: alright"): New tests.
* doc/guix.texi (Invoking guix lint): Mention the 'formatting' checker.