Rename posix_* -> p_* to avoid build conflict on linux

mongoose.h

@@ -596,27 +596,16 @@ void mg_usleep(unsigned long usecs);
 enum { MG_FS_READ = 1, MG_FS_WRITE = 2, MG_FS_DIR = 4 };
 
 // Filesystem API functions
+// stat() returns MG_FS_* flags and populates file size and modification time
+// list() calls fn() for every directory entry, allowing to list a directory
 struct mg_fs {
-  // Return MG_FS_* flags, and populate file size and modification time
   int (*stat)(const char *path, size_t *size, time_t *mtime);
-
-  // Enumerates objects in directory
   void (*list)(const char *path, void (*fn)(const char *, void *), void *);
-
-  // Open file
-  struct mg_fd *(*open)(const char *path, int flags);
-
-  // Close file
-  void (*close)(struct mg_fd *fd);
-
-  // Read file
-  size_t (*read)(void *fd, void *buf, size_t len);
-
-  // Write file
-  size_t (*write)(void *fd, const void *buf, size_t len);
-
-  // Seek file
-  size_t (*seek)(void *fd, size_t offset);
+  struct mg_fd *(*open)(const char *path, int flags);      // Open file
+  void (*close)(struct mg_fd *fd);                         // Close file
+  size_t (*read)(void *fd, void *buf, size_t len);         // Read file
+  size_t (*write)(void *fd, const void *buf, size_t len);  // Write file
+  size_t (*seek)(void *fd, size_t offset);                 // Set file position
 };
 
 // File descriptor