Rename "null" state boolean to "null_terminated_lines" so it is more descriptive and less confusing.

src/include/options.h

@@ -34,7 +34,7 @@ struct opts_s {           /* structure describing run-time options */
 	bool numeric;                  /* numeric output only */
 	bool wait;                     /* wait for transfer before display */
 	bool linemode;                 /* count lines instead of bytes */
-	bool null;                     /* lines are null-terminated */
+	bool null_terminated_lines;    /* lines are null-terminated */
 	bool no_display;               /* do nothing other than pipe data */
 	unsigned long long rate_limit; /* rate limit, in bytes per second */
 	unsigned long long buffer_size;/* buffer size, in bytes (0=default) */

src/include/pv-internal.h

@@ -89,7 +89,7 @@ struct pvstate_s {
 	bool wait;                       /* wait for data before display */
 	bool linemode;                   /* count lines instead of bytes */
 	bool bits;			 /* report bits instead of bytes */
-	bool null;                       /* lines are null-terminated */
+	bool null_terminated_lines;      /* lines are null-terminated */
 	bool no_display;                 /* do nothing other than pipe data */
 	unsigned int skip_errors;        /* skip read errors counter */
 	bool stop_at_size;               /* set if we stop at "size" bytes */

src/include/pv.h

@@ -107,7 +107,7 @@ extern void pv_state_wait_set(pvstate_t, bool);
 extern void pv_state_delay_start_set(pvstate_t, double);
 extern void pv_state_linemode_set(pvstate_t, bool);
 extern void pv_state_bits_set(pvstate_t, bool);
-extern void pv_state_null_set(pvstate_t, bool);
+extern void pv_state_null_terminated_lines_set(pvstate_t, bool);
 extern void pv_state_no_display_set(pvstate_t, bool);
 extern void pv_state_skip_errors_set(pvstate_t, unsigned int);
 extern void pv_state_stop_at_size_set(pvstate_t, bool);