Log Message:
pkgtools/pkglint: update to 5.7.9

Changes since 5.7.8:

* Buildlink3.mk files are checked for typos in the identifier that is
  used for BUILDLINK_TREE, to detect copy-and-paste mistakes.

* Having a rationale is recommended for some variables, especially those
  that make a package fail to build or crash at runtime. This check is
  only active when -Wextra is given, since it is still actively debated
  whether such a check is actually useful.

* Files called Makefile.php can easily be mistaken to be PHP files.
  Therefore the recommended naming convention is to have auxiliary files
  called *.mk. There are already many more files called *.mk than those
  being called Makefile.*.

* The check for unquoted sed substitution commands has been made more
  detailed, but since it is completely disabled, there's nothing to see
  for now.

* The definitions for MASTER_SITE_* are loaded directly from the pkgsrc
  infrastructure instead of hard-coding them in pkglint.