Replace cJSON with mkjson and fix JSON output issues

- Replace cJSON library (3,443 lines) with lightweight mkjson (357 lines)
- Fix mkjson bug: change %Ld to %lld for proper 64-bit integer handling
- Add missing JSON fields: device descriptions, USB3 link states, serial numbers
- Fix USB3 detection by checking hub->super_speed flag instead of port status bits
- Fix buffer allocations with proper size calculations
- Handle all alternate settings in interfaces array
- Replace unsafe string operations (strcpy/strcat) with snprintf
- Change 'speed_bits' field to 'speed_bps' for consistency
- Document JSON output option (-j) in README

Addresses feedback from PR mvp#575. Tested on macOS and Linux.

Author: benroeder

README.md

@@ -224,6 +224,14 @@ are port numbers for all hubs in chain, starting from root hub for a given USB b
 This address is semi-stable - it will not change if you unplug/replug (or turn off/on)
 USB device into the same physical USB port (this method is also used in Linux kernel).
 
+To get the status in machine-readable JSON format, use `-j` option:
+
+    uhubctl -j
+
+This will output status of all hubs and ports in JSON format, making it easy to integrate
+uhubctl with other tools and scripts. The JSON output includes all the same information
+as the text output, including hub info, port status, connected devices, and their properties.
+
 
 Linux USB permissions
 =====================

mkjson.c

@@ -202,7 +202,7 @@ char *mkjson( enum mkjson_container_type otype, int count, ... )
 				else
 					intval = va_arg( ap, long long int );
 
-				if ( allsprintf( chunks + i, "%s%Ld", prefix, intval ) == -1 ) chunks[i] = NULL;
+				if ( allsprintf( chunks + i, "%s%lld", prefix, intval ) == -1 ) chunks[i] = NULL;
 				break;
 
 			// double / long double