maint: prefer STREQ_LEN and STRPREFIX over strncmp in all cases

maint: prefer STREQ_LEN and STRPREFIX over strncmp in all cases

* cfg.mk (sc_prohibit_strncmp): Improve the search pattern: use
_sc_search_regexp to find all invocations of strncmp except when
used on a macro definition line; just match the function name with
an opening parenthesis.  Before, the expression missed places where
the comparison against 0 was in a subsequent line.
* src/system.h (STRNCMP_LIT): Shorten 'literal' to 'lit' to move
the whole definition of the macro into one line - thus making
sc_prohibit_strncmp pass.
(STRPREFIX): Add space before parenthesis.
* src/du.c (main): Prefer STREQ_LEN over strncmp.
* src/pinky.c (scan_entries): Likewise.
* src/tac.c (tac_seekable): Likewise.
* src/who.c (scan_entries): Likewise.