vcard: Fix whitespace handling in line cont's (#9637)

• vcard: Fix whitespace handling in line cont's

Previously, multiple whitespace characters at the start of a
continuation line would all be dropped, instead of only the first one.

Also,

• restrict line continuation characters to SPACE and TAB.

Note that, like before, this identifies the CR (\r) character with the
empty string, and thereby notably does not require a CRLF (\r\n)
sequence (which is mandated by RFCs 2426, 2425) for line termination
(i.e., \n suffices).

Fixes: Bug 1 of issue #9593.

• vcard: Add test for #9593/1

• Fix coding style