POSIX and gcc fixes
Fix several things (separated in 4 commits):
- support explicit_memset(3) (not only explicit_bzero(3);
- gcc < 11 doesn't support either optional arg to __attribute((malloc));
- a POSIX.2 sed(1) requires that the file be the last argument;
- Autest.c includes Xauth.h that requires other xproto headers: add dependency.