Prefer NILP (x) to EQ (x, Qnil)

Prefer NILP (x) to EQ (x, Qnil)

This simplifies the code a bit, and also simplifies some
potential future changes slightly (e.g., altering eq vs eql).
* src/alloc.c (mark_object):
* src/callint.c (fix_command):
* src/chartab.c (Fchar_table_range, Fset_char_table_range):
* src/dbusbind.c (XD_OBJECT_TO_DBUS_TYPE, xd_signature):
* src/dired.c (Fsystem_users):
* src/fileio.c (Fdo_auto_save):
* src/fns.c (concat):
* src/frame.c (get_frame_param, frame_inhibit_resize)
(store_in_alist, store_frame_param, x_set_autoraise)
(x_set_autolower, x_get_arg):
* src/image.c (Fclear_image_cache):
* src/intervals.c (intervals_equal):
* src/intervals.h (DEFAULT_INTERVAL_P):
* src/lread.c (substitute_object_recurse):
* src/menu.c (digest_single_submenu)
(find_and_call_menu_selection)
(find_and_return_menu_selection):
* src/nsfns.m (x_set_icon_name, Fx_create_frame):
* src/nsmenu.m (ns_menu_show):
* src/nsselect.m (ns_string_to_pasteboard_internal)
(Fns_selection_exists_p, Fns_selection_owner_p):
* src/process.c (Faccept_process_output)
(wait_reading_process_output):
* src/terminal.c (store_terminal_param):
* src/textprop.c (verify_interval_modification):
* src/xdisp.c (next_element_from_buffer):
* src/xfaces.c (Finternal_set_lisp_face_attribute):
* src/xfns.c (x_set_icon_type, Fx_synchronize):
* src/xmenu.c (x_menu_show):
* src/xselect.c (Fx_selection_owner_p)
(Fx_selection_exists_p):
* src/xwidget.c (xwidget_view_lookup):
Prefer NILP (x) to EQ (x, Qnil).