support null buffer argument to getcwd, auto-allocating behavior

this is a popular extension some programs depend on, and by using a
temporary buffer and strdup rather than malloc prior to the syscall,
i've avoided the dependency on free and thus minimized the bloat cost
of supporting this feature.

src/unistd/getcwd.c

 #include <unistd.h>
 #include <errno.h>
+#include <limits.h>
+#include <string.h>
 #include "syscall.h"
 
 char *getcwd(char *buf, size_t size)
 {
-	return syscall(SYS_getcwd, buf, size) < 0 ? NULL : buf;
+	char tmp[PATH_MAX];
+	if (!buf) buf = tmp, size = PATH_MAX;
+	if (syscall(SYS_getcwd, buf, size) < 0) return 0;
+	return buf == tmp ? strdup(buf) : buf;
 }