iowatcher: Fix up directory trace processing

iowatcher: Fix up directory trace processing

Similar to the fix for spaces in file names in commit 5d845e3, this
patch fixes processing of directories with spaces in their names by
using posix_spawnp() to run the blkparse command instead of system(). In
doing so, combine_blktrace_devs() and match_trace() have been reworked
to use a list structure instead of doing a lot of strdup()ing and string
appending.

Also make sure that trailing slashes are removed from the directory name
before attempting to use it as the base of the .dump filename.

Update the -t entry in the manpage to mention directory behaviour, too.

Signed-off-by: Andrew Price <anprice@redhat.com>