Executable improvements

• Documentation should be updated, as this makes a number of interface changes.
• RDEPEND should be satisfied for a mod M, prior to DEPEND="M" being considered satisfied for another mod. I.e. if a pybuild has a build dependency on another pybuild, it should require that all runtime dependencies of that dependency are also installed, prior to its own installation. This might be complicated.

In short: It's now possible to install mods that contain executables which can then be used within other pybuilds.

• mod_pretend was added to allow checks to be performed prior to the continue prompt.
• Fixed missing ~ operator in the Atom regex (support was previously added, but it turned out that parsing atoms containing the operator would fail).
• Atoms within the sys-bin category get a small boost to weight if which MN is not empty. In essence, this will prefer sys-bin/MN over bin/MN in dependency strings such as || ( sys-bin/MN bin/MN ), assuming the executable is actually available.
• Fixed a bug where loading a mod using an ambiguous name would cause it to only load the first mod matching the given name.
• Added the version comparison function to the contents of the pybuild module that is accessible within pybuilds.
• mods with PROPERTIES="exec" will get the path defined by EXEC_PATH (in the config) added to the PATH environment variable.