Log Message:
pkgtools/pkglint: update to 5.7.23

Changes since 5.7.22:

* Added a warning for lines that look empty but are actually follow-up
  lines from a previous line.

* Added notes for unusual placement of the continuation backslash. It
  should always be preceded by a single space or tab, or be in column
  73.

* Improved check for needlessly complicated !empty(PKGPATH:Mcat/pkg)
  that is transformed into the simpler ${PKGPATH} == cat/pkg, even if
  the package name contains hyphens, dots, plus or slashes.

* Added check for the _VARGROUPS section since that section contains
  many redundancies that can easily be checked. For example, in
  mk/compiler/gcc.mk the _VARGROUPS section is 67 lines long and
  contains far more than 100 variables. It's tedious to manually check
  this file for internal consistency. That's better left to pkglint.

* The empty variable is no longer flagged as "used but not defined".
  It appears in expressions like ${:Ustring}.

* When --source is combined with --explain, don't unnecessarily
  repeat the source code for a single line if there are several
  diagnostics. Instead, even omit the empty line between the diagnostics
  for the same line.