Added <executable-in-path> and <executable-in-var> bindings

Added <executable-in-path> and <executable-in-var> bindings

These can be used to make a command from a dependency available, either
as a command in $PATH or as a command stored in an environment variable.

For example, if a program requires the "make" command in $PATH:

<requires interface='http://repo.roscidus.com/devel/make'>
  <executable-in-path name='make'/>
</requires>

Or, if it requires the make command in $MAKE:

<requires interface='http://repo.roscidus.com/devel/make'>
  <executable-in-var name='MAKE'/>
</requires>

Note: this required a slight change to the selections XML format.