treewide: fix optional arguments usage

treewide: fix optional arguments usage

In some parts of the code, the optional argument handling is missing
for cases where the argument starts with '='. This is particularly
important for short options with optional arguments, as suggested by
our man pages. The libc getopt_long() handles this for long options,
but for short options, it's our responsibility.

Note that some argument parsing functions (mostly colormode_or_err())
already implement this, as they are usually used with optional
arguments.

Signed-off-by: Karel Zak <kzak@redhat.com>