pgrep: add warning that pattern exceeeds 15 chars

Add a warning if you specify a command over 15 characters and don't
use the -f command.

This is a pick of two patches from master:
 24fd260 pgrep: Fix off by one error in line check
 4a7f9fc pgrep - adds warning that pattern exceeds 15 chars without

References:
 !25

pgrep.c

@@ -830,7 +830,14 @@ static void parse_opts (int argc, char **argv)
     }
 
     if (argc - optind == 1)
+    {
         opt_pattern = argv[optind];
+        if ((!opt_full) && (strlen(opt_pattern) > 15))
+            xwarnx(_("pattern that contains program name longer than 15 characters will result in zero matches\n"
+                     "Try `%s -f' option for thorough search."),
+                   program_invocation_short_name);
+    }
+
     else if (argc - optind > 1)
         xerrx(EXIT_USAGE, _("only one pattern can be provided\n"
                      "Try `%s --help' for more information."),