common/libexec: remove redundant pre/do/post preparation code

This pullrequest removes redundant codepaths in
xbps-src-do{build,configure,install}.sh and joins the code in the
run_step function. This causes slightly different behavior to
do_install:
Do install will chdir to wrksrc only _before_ the first step. The
current behavior is that pre_install will run without a chdir, do_ and
post_ is runned with a chdir. This is a subtle but breaking change and
may cause some templates to break at install phase.

common/xbps-src/libexec/xbps-src-doinstall.sh

@@ -37,32 +37,7 @@ if [ "$SUBPKG_MODE"  = "no" ]; then
     if [ ! -f $XBPS_INSTALL_DONE ] || [ -f $XBPS_INSTALL_DONE -a -n "$XBPS_BUILD_FORCEMODE" ]; then
         mkdir -p $XBPS_DESTDIR/$XBPS_CROSS_TRIPLET/$pkgname-$version
 
-        # Run pre-install hooks
-        run_pkg_hooks pre-install
-
-        # Run pre_install()
-        if declare -f pre_install >/dev/null; then
-            run_func pre_install
-        fi
-
-        # Run do_install()
-        cd "$wrksrc"
-        [ -n "$build_wrksrc" ] && cd $build_wrksrc
-        if declare -f do_install >/dev/null; then
-            run_func do_install
-        else
-            if [ ! -r $XBPS_BUILDSTYLEDIR/${build_style}.sh ]; then
-                msg_error "$pkgver: cannot find build helper $XBPS_BUILDSTYLEDIR/${build_style}.sh!\n"
-            fi
-            . $XBPS_BUILDSTYLEDIR/${build_style}.sh
-            run_func do_install
-        fi
-        # Run post_install()
-        cd "$wrksrc"
-        [ -n "$build_wrksrc" ] && cd $build_wrksrc
-        if declare -f post_install >/dev/null; then
-            run_func post_install
-        fi
+        run_step install "" skip
 
         touch -f $XBPS_INSTALL_DONE
     fi