0/4: syntax-check: tweak for fewer false positives

[Jim Meyering]

Richard W.M. Jones went through the (tedious!) process of reviewing
all of libguestfs wrt gnulib's latest syntax-check rules, and reported
many false positives.  In a few cases, I found ways to avoid entire
classes of false positives:

  1/4 maint.mk: relax sc_prohibit_strcmp, to avoid a false positive
  2/4 maint.mk: teach sc_prohibit_magic_number_exit to accept 77
  3/4 maint.mk: sc_prohibit_path_max_allocation: don't FP for UNIX_PATH_MAX
  4/4 maint.mk: exempt trailing blanks found in "binary" files