maint: prohibit direct use of strncmp: prefer STREQ_LEN, STRNCMP_LIT

maint: prohibit direct use of strncmp: prefer STREQ_LEN, STRNCMP_LIT

* cfg.mk (sc_prohibit_strncmp): New rule, mostly from libvirt.
* src/system.h (STREQ_LEN, STRPREFIX, STRNCMP_LIT): Define.
* src/df.c (get_dev, get_point): Convert.
* src/extent-scan.c (extent_need_sync): Likewise.
* src/ls.c (is_colored, decode_switches): Likewise.
(parse_ls_color, (print_color_indicator): Likewise.
* src/md5sum.c (split_3): Likewise.
* src/split.c (main, emit_ancillary_info): Likewise.
* src/tr.c (look_up_char_class): Likewise.
* src/uname.c (main): Likewise.
* src/who.c (scan_entries): Likewise.