The current gdb patch to support NIX_DEBUG_INFO_DIRS fails if
the user attempts to change the debug file directory to a
value other than the default. For instance:
(gdb) set debug-file-directory /run/booted-system/sw/lib/debug
results in:
munmap_chunk(): invalid pointer
Aborted (core dumped)
To fix this issue, the debug_file_directory is allocated
with xstrdup so that a subsequent call to xfree will succeed.
