Also removed mentions from most docs.

Discussed: https://curl.haxx.se/mail/lib-2019-05/0045.html

Closes #3888

scan-build would warn on a potential access of an uninitialized
buffer. I deem it a false positive and had to add this somewhat ugly
work-around to silence it.

... since the 'tv' stood for timeval and this function does not return a
timeval struct anymore.

Also, cleaned up the Curl_timediff*() functions to avoid typecasts and
clean up the descriptive comments.

Closes #2011

... to cater for systems with unsigned time_t variables.

- Renamed the functions to curlx_timediff and Curl_timediff_us.

- Added overflow protection for both of them in either direction for
  both 32 bit and 64 bit time_ts

- Reprefixed the curlx_time functions to use Curl_*

Reported-by: Peter Piekarski
Fixes #2004
Closes #2005

... to make all libcurl internals able to use the same data types for
the struct members. The timeval struct differs subtly on several
platforms so it makes it cumbersome to use everywhere.

Ref: #1652
Closes #1693

... as it does extra checks to actually work.

Reported-by: jonrumsey at github
Fixes #1504

When the random seed is purposely made predictable for testing purposes
by using the CURL_ENTROPY environment variable, process that data in an
endian agnostic way so the the initial random seed is the same
regardless of endianness.

- Change Curl_rand to write to a char array instead of int array.

- Add Curl_rand_hex to write random hex characters to a buffer.

Fixes #1315
Closes #1468

Co-authored-by: Daniel Stenberg
Reported-by: Michael Kaufmann

Closes #1356