tool_getparam: clear sensitive arguments better

tool_getparam: clear sensitive arguments better

curl attempts to clear some flags to hide them from snooping neighbors
(on platforms where it works). For example the credentials provided with
-u. Previously it would only do that if there was a space between the
option and the credentials as in "-u joe:s3cr3t" but not when done
without a separating space as in "-ujoe:s3cr3t".

This addresses that previous shortcoming.

Reported-by: kayrus on github
Fixes #16396
Closes #16401