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

testsuite/pgrep.test/pgrep.exp

@@ -118,6 +118,10 @@ set test "pgrep does not match substring with exact"
 spawn $pgrep -x $testproc_trim
 expect_blank $test
 
+set test "pgrep with long match gives warning"
+spawn $pgrep gnome-session-bi
+expect_pass "$test" "pattern that contains program name longer than 15 characters will result in zero matches"
+
 
 # Cleanup
 kill_testproc