doc: Prefer https urls where possible.
* doc/gnulib.texi (POSIXURL): Use https url.
* doc/multithread.texi: Use https instead of http where possible.
* doc/glibc-functions/*.texi: Likewise.
* doc/posix-functions/*.texi: Likewise.
* lib/glthread/lock.c: Likewise.
* lib/hamt.h: Likewise.
* lib/localcharset.c: Likewise.
* lib/pipe2.c: Likewise.
* lib/pipe.c: Likewise.
* lib/sigsegv.c: Likewise.
* lib/sigsegv.in.h: Likewise.
* lib/sm3.c: Likewise.
* lib/sm3.h: Likewise.
* lib/sm3-stream.c: Likewise.
* lib/stackvma.c: Likewise.
* lib/stdio-impl.h: Likewise.
* lib/stdnoreturn.in.h: Likewise.
* m4/getopt.m4: Likewise.
* m4/largefile.m4: Likewise.
* m4/stdalign.m4: Likewise.
* STATUS-libposix: Likewise.
* tests/test-float.c: Likewise.
* tests/test-getopt.h: Likewise.
* tests/test-setenv.c: Likewise.
* tests/test-spawn-pipe-child.c: Likewise.
* tests/test-strerror.c: Likewise.
* tests/test-strerror_r.c: Likewise.
* tests/test-unsetenv.c: Likewise.
* users.txt: Likewise.