Introduce common/hooks to implement hooks at specified phases.

xbps-src will soon move some stuff to the common hooks directory.

common/hooks/README

@@ -0,0 +1,27 @@
+HOOKS
+=====
+
+This directory contains shell hooks that are processed after or before the
+specified phase. The shell hooks are simply shell snippets (must not be
+executable nor contain a shebang) that are processed lexically by xbps-src.
+Only files with the `.sh` extension are processed.
+
+The following directories are used to set the order in which the hooks
+should be processed by xbps-src:
+
+	* pre-fetch		(before running fetch phase)
+	* post-fetch		(after running fetch phase)
+	* pre-extract		(before running extract phase)
+	* post-extract		(after running extract phase)
+	* pre-configure		(before running configure phase)
+	* post-configure	(after running configure phase)
+	* pre-build		(before running build phase)
+	* post-build		(after running build phase)
+	* pre-install		(before running install phase)
+	* post-install		(after running install phase)
+	* pre-pkg		(before running pkg phase)
+	* post-pkg		(after running pkg phase)
+
+NOTES
+~~~~~
+* Symlinks can be created (relative) to make a hook available in multiple phases.