8571 Makefile.master should not trust $PATH

*** 132,144 ****
  STABS=          $(ONBLD_TOOLS)/bin/$(MACH)/stabs
  ELFEXTRACT=     $(ONBLD_TOOLS)/bin/$(MACH)/elfextract
  MBH_PATCH=      $(ONBLD_TOOLS)/bin/$(MACH)/mbh_patch
  BTXLD=          $(ONBLD_TOOLS)/bin/$(MACH)/btxld
  VTFONTCVT=      $(ONBLD_TOOLS)/bin/$(MACH)/vtfontcvt
  ECHO=           echo
- INS=            install
  TRUE=           true
  SYMLINK=        /usr/bin/ln -s
  LN=             /usr/bin/ln
  CHMOD=          /usr/bin/chmod
  MV=             /usr/bin/mv -f
  RM=             /usr/bin/rm -f
--- 132,151 ----
  STABS=          $(ONBLD_TOOLS)/bin/$(MACH)/stabs
  ELFEXTRACT=     $(ONBLD_TOOLS)/bin/$(MACH)/elfextract
  MBH_PATCH=      $(ONBLD_TOOLS)/bin/$(MACH)/mbh_patch
  BTXLD=          $(ONBLD_TOOLS)/bin/$(MACH)/btxld
  VTFONTCVT=      $(ONBLD_TOOLS)/bin/$(MACH)/vtfontcvt
+ # echo(1) and true(1) are specified without absolute paths, so that the shell
+ # spawned by make(1) may use the built-in versions.  This is minimally
+ # problematic, as the shell spawned by make(1) is known and under control, the
+ # only risk being if the shell falls back to $PATH.
+ #
+ # We specifically want an echo(1) that does interpolation of escape sequences,
+ # which ksh93, /bin/sh, and bash will all provide.
  ECHO=           echo
  TRUE=           true
+ INS=            $(ONBLD_TOOLS)/bin/$(MACH)/install
  SYMLINK=        /usr/bin/ln -s
  LN=             /usr/bin/ln
  CHMOD=          /usr/bin/chmod
  MV=             /usr/bin/mv -f
  RM=             /usr/bin/rm -f