darcs patch: Search for programs in bindir of user

Hi, I wrote this for #663. Unfortunately, I did not find an easy way to find out if a Program type is using the standard way of locating a program or if it uses a custom method for this (without breaking the signitures). So my solution tries to locate a program (no compilers) first in the directories of PATH and then falls back to the bindir that will be used to install the program later on. Not sure if this is the nicest way to do this, since program detection has to be split up in programs that are found the traditional way and others that are additionally search in the bindir. Cheers, Martin Sun Nov 7 15:13:12 CET 2010 Martin Kiefel * Search for programs in bindir of user See Ticket #663. If program is not found in the PATH directories it will be search in the local bindir, which was specified by the user.