Added mmap-based implementation for POSIX-compliant systems.

[ghost]

Preamble

Tar is a very simple format, which makes it possible to very easily find a pointer to a packed file.
Using mmap, I added the ability to directly map a tar file into the computer's address space, and
find pointers to large pieces of memory.

This eliminates potentially unnecessary copies, and in my testing (getting the contents of a file many
thousands of times), the mmap-based implementation proved to be 60% faster, which is a great improvement.

What was added

I added mtar_open_mapped,mtar_get_mapped, and mtar_get_pointer
to open, query, and loop through a mapped file respectively.

What was lost

All features were added, no functionality was lost in the base API.

Example

mtar_t tar;
const char *str;
mtar_header_t h;

/* Open the file and map its contents */
mtar_open_mapped(&tar, "test.tar");

/* Print all file names and sizes */
while ( (mtar_read_header(&tar, &h)) != MTAR_ENULLRECORD ) {
  const char *data;
  printf("%s (%d bytes)\n", h.name, h.size);
  /* Get file pointer and bump to next header */
  mtar_get_pointer(&tar, (const void**) &data);
}

/* Get mapped pointer to "test.txt" */
mtar_get_mapped(&tar, "test.txt", &str);
/* Directly print contents */
printf("%s", str);
mtar_close(&tar);

[galsjel]

info is never freed.

[galsjel]

You should be able to get away with initializing this on the stack, I don't see a purpose for allocating if it never leaves this scope.