lib: prefer `var = time(NULL)` over `time(&var)`

lib: prefer `var = time(NULL)` over `time(&var)`

Following up on previous occurrences showing up as gcc warnings, replace
the remaining `time(&var)` calls with `var = time(NULL)`, though these
aren't specifically causing compiler warnings. These are in the TFTP
client code (`lib/tftp.c`), except one which is in a debug branch in
`lib/http_aws_sigv4.c`.

What's unexplainable is that this patch seems to mitigate TFTP tests
often hanging or going into an infinite loop on GHA windows workflows
with MSYS2, mingw-w64 and MSVC (Cygwin is unaffected):
  https://github.com/curl/curl/pull/13599#issuecomment-2119372376
TFTP hangs did not entirely disappear though, so could be unrelated.

`time()` docs:
https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/time-time32-time64
https://manpages.debian.org/bookworm/manpages-dev/time.2.en.html

Follow-up to 58ca0a2f0743a586716ca357c382b29e3f08db69 #13800
Follow-up to d0728c9109629ee82b855b350a4c3f1f52ee61df #13643
Closes #13815