Minor backslash fixes in manuals and scripts

Minor backslash fixes in manuals and scripts

* Makefile.in (install-arch-indep):
* admin/charsets/compact.awk:
* admin/charsets/gb180302.awk (gb_to_index):
* admin/charsets/gb180304.awk (gb_to_index):
Avoid undefined behavior in Awk regular expression backslashes.
* doc/misc/efaq.texi (Matching parentheses):
Omit unnecessary backslashes.
* doc/misc/gnus-faq.texi (FAQ 5-8):
Avoid undefined behavior in suggested sed backslash usage.