Set enviroment variable PAGER=pager if using option `man -P pager'

This inherit variable PAGER for childs like whatis(1), e.g.
`man -P less -k man' should now work like expected.  Close PR#1068

gnu/usr.bin/man/man/man.c

@@ -315,6 +315,8 @@ man_getopt (argc, argv)
 	  break;
 	case 'P':
 	  pager = strdup (optarg);
+	  if (setenv("PAGER", pager, 1) != 0)
+		  (void)fprintf(stderr, "setenv PAGER=%s\n", pager);
 	  break;
 	case 'S':
 	  colon_sep_section_list = strdup (optarg);