Add simple comparisons in formats: #{==:a,b} and #{!=:a,b} ("a" and "b"

are expanded so can compare formats). And expand the condition to
#{?a,b,c} (the "a" part) if it doesn't work as a simple lookup.

Also add FORMAT_NOJOBS flag to disable jobs in a format.

tmux.h

@@ -1568,6 +1568,7 @@ char		*paste_make_sample(struct paste_buffer *);
 /* format.c */
 #define FORMAT_STATUS 0x1
 #define FORMAT_FORCE 0x2
+#define FORMAT_NOJOBS 0x4
 struct format_tree;
 struct format_tree *format_create(struct cmdq_item *, int);
 void		 format_free(struct format_tree *);