maint: indent with spaces, not TABs, and add a rule to check this * tests/test-userspec.c: Indent with spaces, not TABs. * tests/test-argp.c: Likewise. * tests/test-c-stack2.sh: Likewise. * tests/test-parse-duration.sh: Likewise * m4/strtod.m4: Likewise. * m4/alloca.m4: Likewise. * m4/pselect.m4: Likewise. * Makefile (sc_prohibit_leading_TABs): Prohibit leading TABs.