Use intern_c_string instead of intern.

Use intern_c_string instead of intern.
* src/nsselect.m (syms_of_nsselect):
* src/nsmenu.m (syms_of_nsmenu):
* src/nsfns.m (syms_of_nsfns):
* src/msdos.c (syms_of_msdos):
* src/image.c (syms_of_image):
* src/charset.c (syms_of_charset): Use intern_c_string instead of intern.