dm-latency: several bug fixes to lib_dm_io_latency.c

Fixes several bugs in lib_dm_io_latency.c including,
1, in check_parameters() uses strdup() to copy a string, then call
basename(), because basename() may change target_name parameter.
2, fix a return value typo in load_system_default_configs()
3, modify printf debug information, only call printf when DEBUG is
defined.
4, in is_dm_target_io_latency_ok(), if there are errors, returns -1.
dm-latency: demo program to use lib_dm_io_latency

Add a demo program latency_warning.c to show how to use
lib_dm_io_latency library.