Clean mailmbox plugin warnings
Cleans:
• plugin_gtk.c:312:9: warning: variable 'old_path' set but not used
• plugin_gtk.c:397:9: warning: variable 'old_path' set but not used
• mailimf_write.c:636:16: warning: variable 'next_word' set but not used
• mailimf_write.c:635:16: warning: variable 'word_end' set but not used
• mailmbox.c:519:10: warning: variable 'fixed_size' set but not used
• mailmbox.c:610:10: warning: variable 'maxuid' set but not used
• mailmbox_folder.c:162:36: warning: assignment from incompatible pointer type
• mailmbox_folder.c:872:5: warning: 'r' may be used uninitialized in this function