SEL_CDHOME, SEL_CDBEGIN, SEL_CDLAST, SEL_VISIT

According to manpage of strcasestr [1], the macro function needs to be added.

[1] https://linux.die.net/man/3/strcasestr

Signed-off-by: SZ Lin (林上智) <szlin@debian.org>

It seems the behaviour of the default opener (xdg-open) varies across platforms
and environments. Details are documented in the issue. To prevent an empty file
from being accidentally opened leading to unexpected behaviour, we are adding a
guard with the message that the empty file should be opened using edit or open
option.