scope-tightening changes

[Jim Meyering]

There were a bunch of unnecessarily global functions and variables in
gzip's sources.  In the course of reducing their scope I found at least
one variable that was not used at all, and ended up removing the unused
crypt.[ch] files.

These changes were inspired by the new-to-gnulib tight_scope syntax-check
rule.  Now that it's enabled, if someone accidentally declares a new
variable or function without the static attribute, "make syntax-check"
will fail.

[PATCH 1/7] maint: prepare for tight-scope rule: use noinst_HEADERS
[PATCH 2/7] maint: remove all uses of OF((...)) prototype-hiding
[PATCH 3/7] maint: limit scope of several functions
[PATCH 4/7] maint: remove crypt.[ch] stubs
[PATCH 5/7] maint: declare many variables to be static
[PATCH 6/7] maint: prepare for gnulib's new tight-scope syntax-check
[PATCH 7/7] build: update gnulib submodule to latest