Allow unrecognized options in config file

[cbeams]

Prior to this commit, Bisq's new Config infrastructure would throw an
exception on encountering an unrecognized option in the
bisq.properties config file and the Bisq application would handle this
by reporting an error at the command line indicating that the given
option was not recognized.

For example, given a bisq.properties file that reads as follows:

$ cat $APP_DATA_DIR/bisq.properties
bogus=42

running Bisq would exit with the following error message:

$ java -jar bisq.jar
error: problem parsing option 'bogus': bogus is not a recognized option.

This was reasonable enough behavior, but it had two problems. The first
is that the error message did not indicate to the user that the
unrecognized option was found in the config file as opposed to the
command line. Users unfamiliar with the config file might not know to
look there to eliminate or fix the offending option. The second problem
surfaced when testing the Windows executable build for the v1.2.6
release. When such an error was encountered, the executable would just
fail silently, reporting nothing to the user.

Both of these problems should be addressed on their own, i.e. option
parsing errors should report to the user whether the offending option
was at the command line or in the config file, and our packaged
executables should never just fail silently if we can avoid it.

However, this change does not address either of these problems directly.
It rather avoids the problems completely by relaxing config file
parsing to silently allow unrecognized options. This behavior mimics
prior (pre-Config) Bisq versions, and it also follows suit with
Bitcoin Core's own config file processing.

This fixes #3966, which surfaced this problem. In that particular issue,
an option line had somehow been added to the user's bisq.properties
config file that consisted of an option key containing 452 octal null
characters (\u0000) followed by an equals sign and an empty option
value. It is unknown how this bogus option was ever added to the user's
config file in the first place, but on the chance that previous Bisq
versions somehow added this, the changes in this commit ensure that such
entries will not cause affected users' Bisq applications to silently
fail in versions 1.2.6 and beyond.

[ripcurlx]

ACK

Tested it on Regtest with a broken bisq.properties file and it behaves as in v1.2.5 now.