Move system_exec from gtk2_ardour/ to libs/pbd/
Add command-line parsing constructor to SystemExec

Add a new constructor to SystemExec which takes a complete command-line
and a map of substitutions for %<char>, and parses the command line,
taking account of some simple escape sequences and quoting.
Use SystemExec for post-export hook

Use the new command-line parsing constructor for SystemExec to construct
the args array for the post-export hook from the entered command string,
with some simple substitutions for filename, directory, &c.