Allow use of :filter syntax in udf filter commands

For example "command conf filter/.*\.cfg$/".
Add more unit-tests for regex args of engine/cmds
Make filter_t parameter of filter_is_empty() const
Formatted output of :fitler? as a table
Merge branch 'filter-follow-up-changes'

Allow use of :filter syntax in user-defined filter commands (e.g.
"command conf filter/.*\.cfg$/").

Format output of :filter? command as a table.

Thanks to filterfalse.
Simplify status checks in fuse.c

No need to check reverse condition in RHS of an OR-expression, we get
there only if LHS is false, which means that !LHS is true.
Make process_cancel_request() extern in utils

Will be used to omit duplication in background.c
Use pid_t in background_and_wait_for_status()
Reduce var scope in background_and_wait_for_status
Add handling of Ctrl-C for FUSE mounting

But not unmounting, which is dangerous as it might cause data loss.
Merge branch 'allow-cancelling-fuse-mounting'

Add cancellation (handling of Ctrl-C) for FUSE mounting (but not
unmounting).