pathtrace.c: introduce user-provided sets of paths

pathtrace.c: introduce user-provided sets of paths

* defs.h (struct path_set): New structure.
(global_path_set): New variable prototype.
(tracing_paths): Change macro body to use global_path_set variable.
(pathtrace_select_set): Add "struct path_set *" argument.
(pathtrace_match_set): Likewise.  Change return type to bool.
(pathtrace_select, pathtrace_match): Change into thin macro wrappers
around pathtrace_select_set and pathtrace_match_set, repsectively.
* pathtrace.c (global_path_set): New variable.
(storepath, pathtrace_select_set): Add "struct path_set *" argument.
(pathmatch, upathmatch, fdmatch, pathtrace_match_set): Likewise.
Change return type to bool.