maint: support 5.3.0 integers correctly.

Many integer fields and values were incorrectly becoming `.0`
suffixed floats in Lua 5.3.0.
* ext/include/_helpers.c (lua_isinteger): Define to lua_isnumber
with pre-5.3.0 Lua headers.
(checkinteger): Use it.
(checkch): Use checkint over luaL_checknumber.
(checkintegerfield): Use lua_pushinteger over lua_pushnumber.
(pushnumberfield, setnumberfield): Rename from these...
(pushintegerfield, setintegerfield): ...to these, and adjust to
work with Integer subtypes in Lua 5.3.0.
* ext/posix/curses.c (Punctrl): Use checkch over luaL_checknumber.
* ext/posix/grp (pushgroup): Set gr_gid field as an integer.
* ext/posix/poll.c (poll_fd_list_check_table): Use lua_isinteger
for argument check.
* ext/posix/pwd.c (pushpasswd): Use setintegerfield for pw_uid
and pwd_gid fields.
* ext/posix/sys/resource.c (pushrlimit): Use setintegerfield for
rlim_cur and rlim_max fields.
* ext/posix/sys/socket.c (pushsockaddrinfo): Use pushintegerfield
for family, port, pid and groups fields.
(Pgetaddrinfo): Use pushintegerfield for socktype and protocol
fields.
* ext/posix/sys/stat.c (pushstat): Use setintegerfield for all
fields.
* ext/posix/sys/statvfs.c (pushstatvfs): Use setintegerfield for
all fields.
* ext/posix/sys/time.c (pushtimeval): Use setintegerfield for
tv_sec and tv_usec fields.
* ext/posix/sys/times.c (pushtimefield): Use pushintegerfield.
* ext/posix/termio.c (Ptcgetattr): Use pushintegerfield for
iflag, oflag, lflag, cflag and cc fields.
* ext/posix/time.c (pushtimespec): Use setintegerfield for
tv_sec and tv_nsec fields.
(pushtm): Use setintegerfield for all fields.
* ext/posix/unistd.c (mygetuid. mygetgid): Use lua_isinteger and
lua_tointeger for argument checking.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>