avoid misc. warnings

avoid misc. warnings

* lib/fsusage.c (UNUSED_PARAM): Define.
(get_fs_usage): Mark parameter "disk" as unused.
* lib/getugroups.c (getgrent): Use "void" in prototype.
* lib/mountlist.c: Mark unused parameters.
(read_file_system_list): Declare a local with "const".
* lib/nanosleep.c (getnow): Declare static.
* lib/strftime.c: Include strftime.h, for declaration of nstrftime.