lid: avoid reading beyond end of buffer for a long name

* libidu/idfile.h (stzncpy): Define, from coreutils.
* src/lid.c (query_ambiguous_prefix): Avoid buffer overrun.
Using strncpy to copy a too-long name would result in a "name"
that is not NUL-terminated, yet that name would be treated as
a NUL-terminated string immediately afterwards, via report_func,
which attempts to print it.
* libidu/fnprint.c (root_name): Use stzncpy in place of strncpy.
* NEWS (Bug fixes): Mention the bug fix.
build: default to --enable-gcc-warnings in a git tree

Anyone building from cloned sources can be assumed to have a new
enough environment, such that enabling gcc warnings by default will
be useful.  Tarballs still default to no warnings, and the default
can still be overridden with --disable-gcc-warnings.
* configure.ac (gl_gcc_warnings): Set default based on environment.
build: update gnulib, bootstrap and init.sh
doc: remove idutils.texi front- and back-cover text requirements

* doc/idutils.texi: Do not require front- or back-cover text,
thus making this section the same as gzip's, so this may be
included in Debian's "free" (rather than non-free) archive.
doc: change the manual to build with texinfo 5.1

* doc/idutils.texi (Top): Rearrange menu.
(Quick start): Use @itemize and @item.
(Common options): Rearrange menu.
(Walker options): Use @item, not @itemx.
(Past and Future): Use @itemize and @item.
maint: update copyright years
gnulib: update to latest