Decast nxt_cpymem()

nxt_cpymem() is basically mempcpy(3)

Like mempcpy() nxt_cpymem() returns a void *.

nxt_cpymem() is implemented as a wrapper around memcpy(3), however
before returning the new pointer value we cast the return of memcpy(3)
to a u_char *, then add the length parameter to it.

I guess this was done to support compilers that do not support
arithmetic on void pointers as the C standard forbids it.

However since we removed support for compilers other than GCC and Clang
(ending in commit 9cd11133 ("Remove support for Sun's Sun Studio/SunPro
C compiler")) this is no longer an issue as both GCC and Clang support
arithmetic on void pointers (without the -pedantic option) by treating
the size of a void as 1.

While removing the unnecessary casting in this case doesn't necessarily
improve type-safety (as we're dealing with void *'s in and out), it does
just make the code that little more readable.

Oh and for interest we have actually already been relying on this
extension

  src/nxt_array.c:143:40: warning: arithmetic on a pointer to void is a GNU extension [-Wgnu-pointer-arith]
    143 |             nxt_memcpy(data, src->elts + (i * size), size);
        |                              ~~~~~~~~~ ^
  src/nxt_string.h:45:24: note: expanded from macro 'nxt_memcpy'
     45 |     (void) memcpy(dst, src, length)
        |                        ^~~

which was introduced in e2b53e16 ("Added "rootfs" feature.") back in
2020.

Link: <https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html>
Link: <https://clang.llvm.org/docs/LanguageExtensions.html#introduction>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

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