The 'check_port_source' checks whether all remote sources defined for a given
port are currently available. It returns zero, when all remote resources are
available.
Fix#1430
The patch supports both, a download-specific UNZIP_OPT(download) and a
general UNZIP_OPT that can be defined across downloads.
UNZIP_OPT(download) overrides UNZIP_OPT.
Note, the `--strip-components=1` argument is not required for unzip.
Issue #1357
Some downloads are available via HTTPS only, but wget < 3.14 does not
support server-name identification, which is used by some sites. So, we
disable certificate checking in wget and check the validity of the
download via SIG or SHA.
Fixes#1334.