Fix temporary and bookmark file names with slashes

Some of the bookmarks can have slashes in the name. For example,
"Microsoft-Windows-Sysmon/Operational". The slashes can get interpreted
as part of the path when creating both the bookmark and the temporary
files used to update the bookmarks which results in errors.

There was already some code to replace the last slash in the
eventchannel name with a space when creating the bookmark file name.
This code was turned into a function called replace_slash() and is now
also done when creating the bookmark temporary file names. The
temprorary file names were also changed slightly by adding a trailing
'-' before the random characters are added by mkstemp_ex().

Also, the slash was replaced with a space. A space in file names is sort
of ugly so that was changed to an underscore. It may have some affect
on anyone using eventchannel and future events but hopefully the impact
is negligible.