Convert mbmesh module from C to C++ syntax

[Copilot]

The mbmesh module was using C syntax with .c file extensions. This converts it to proper C++ while maintaining compatibility with MB-System's C libraries.

Changes

File extensions and build

• Renamed .c → .cpp (4 files)
• Updated Makefile.am sources list

C++ syntax conversion

• Replaced C headers with C++ equivalents: stdio.h → cstdio, stdlib.h → cstdlib, string.h → cstring, math.h → cmath, time.h → ctime
• Replaced NULL with nullptr (11 occurrences)
• Added extern "C" wrapper in mbmesh.h for MB-System C library linkage

Example change:

// Before
#include <stdio.h>
FILE *fp = fopen(filename, "r");
if (fp == NULL) { ... }

// After  
#include <cstdio>
FILE *fp = fopen(filename, "r");
if (fp == nullptr) { ... }

Memory management remains malloc/free based since the code interfaces with MB-System's C libraries through the extern "C" boundary.

Original prompt

I realized that all the files in src/mbmesh are all in c and not C++. Are you able to fix that?

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