libcurl/opts: do not save files in dirs where attackers have access

libcurl/opts: do not save files in dirs where attackers have access

libcurl cannot fully protect against attacks where an attacker has write
access to the same directory where it is directed to save files. This is
particularly sensitive if you save files using elevated privileges.

Previously only mentioned in VULN-DISCLOSURE-POLICY.md.

Highlighted-by: Donguk Kim
Closes #16051