maint: adjust cpp indentation to reflect nesting depth

maint: adjust cpp indentation to reflect nesting depth

I.e., in a block of code that begins with an unnested "#if",
put one space between the "#" in column 1 and following token.
For example,
-#include <sys/vfs.h>
+# include <sys/vfs.h>
Do this only in .c files that are part of a module I maintain.
* lib/linkat.c: Filter through cppi.
* lib/nanosleep.c: Likewise.
* lib/openat.c: Likewise.
* lib/openat-die.c: Likewise.
* lib/dup3.c: Likewise.
* lib/fchownat.c: Likewise.
* lib/flock.c: Likewise.
* lib/fsync.c: Likewise.
* lib/fts.c: Likewise.
* lib/getpass.c: Likewise.
* lib/gettimeofday.c: Likewise.
* lib/userspec.c: Likewise.
* Makefile (sc_cpp_indent_check): New rule, to check this.