compat: move strdup(3) replacement to its own file

Move our implementation of strdup(3) out of compat/nedmalloc/ and
allow it to be used independently from USE_NED_ALLOCATOR.  The
original nedmalloc doesn't come with strdup() and doesn't need it.
Only _users_ of nedmalloc need it, which was added when we imported
it to our compat/ hierarchy.

This reduces the difference of our copy of nedmalloc from the
original, making it easier to update, and allows for easier testing
and reusing of our version of strdup().

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/strdup.c

@@ -0,0 +1,11 @@
+#include "../git-compat-util.h"
+
+char *gitstrdup(const char *s1)
+{
+	size_t len = strlen(s1) + 1;
+	char *s2 = malloc(len);
+
+	if (s2)
+		memcpy(s2, s1, len);
+	return s2;
+}