Implement @gettext<...@> directive to call gettext from skeletons. It

Implement @gettext<...@> directive to call gettext from skeletons.  It
can be used anywhere including inside arguments of directives of the
form @foo(...@).  Eventually, it should be possible to write an
xgettext-like utility to extract @gettext<...@> text from skeletons.
* data/bison.m4 (b4_check_user_names): Use @gettext<...@> when invoking
b4_warn_at since b4_warn_at doesn't invoke gettext anymore.
* data/glr.cc, data/lalr1.cc: Likewise but for b4_fatal and
b4_complain reporting a push-parsing request or a missing %defines.
* src/scan-skel.l (INITIAL): Recognize `@gettext<' to start...
(SC_AT_GETTEXT_ARG): ... this new start condition where gettext is
invoked.
(SC_AT_DIRECTIVE_ARG): Rename to...
(SC_AT_DIRECTIVE_ARGS): ... this, and recognize `@gettext<' to start
SC_AT_GETTEXT_ARG.
(at_directive_perform): Don't invoke gettext for @warn(...@),
@warn_at(...@), etc.  Instead, let the skeletons decide what should be
passed to @gettext<...@>.
* tests/skeletons.at (@gettext<...@>): New test case.

* tests/skeletons.at (installed skeleton file name): Rename to...
(installed skeleton file names): ... this.