gstrfuncs: Add g_memdup2() function

This will replace the existing `g_memdup()` function, which has an
unavoidable security flaw of taking its `byte_size` argument as a
`guint` rather than as a `gsize`. Most callers will expect it to be a
`gsize`, and may pass in large values which could silently be truncated,
resulting in an undersize allocation compared to what the caller
expects.

This could lead to a classic buffer overflow vulnerability for many
callers of `g_memdup()`.

`g_memdup2()`, in comparison, takes its `byte_size` as a `gsize`.

Spotted by Kevin Backhouse of GHSL.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
Helps: GHSL-2021-045
Helps: #2319

docs/reference/glib/glib-sections.txt

@@ -1341,6 +1341,7 @@ g_newa
 <SUBSECTION>
 g_memmove
 g_memdup
+g_memdup2
 
 <SUBSECTION>
 GMemVTable