Introduce astcachedir, to be used for temporary bucket files

As described in the issue, /tmp is not a suitable location for a
large amount of cached media files, since most distributions make
/tmp a RAM-based tmpfs mount with limited capacity.

I opted for a location that can be configured separately, as opposed
to using a subdirectory of spooldir, given the different storage
profile (transient files vs files that might stay there indefinitely).

This commit just makes the cache directory configurable, but leaves
it at /tmp by default, to ensure backwards compatibility.

A future commit that only targets master could change the default
location to something more sensible such as /var/tmp/asterisk. At
that point, the cachedir could be created and cleaned up during
uninstall by the Makefile script.

ASTERISK-29143

Change-Id: Ic54e95199405abacd9e509cef5f08fa14c510b5d

build_tools/install_subst

@@ -26,6 +26,7 @@ trap cleanup_tmp 0
 src="$1"
 dst="$2"
 sed <"$src" \
+	-e "s|__ASTERISK_CACHE_DIR__|$ASTCACHEDIR|g" \
 	-e "s|__ASTERISK_DATA_DIR__|$ASTDATADIR|g" \
 	-e "s|__ASTERISK_DB_DIR__|$ASTDBDIR|g" \
 	-e "s|__ASTERISK_ETC_DIR__|$ASTETCDIR|g" \