base-files: various enhancements to network.sh

base-files: various enhancements to network.sh

	- support reading inactive gateways and DNS information in
	  network_get_gateway(), network_get_dnsserver() and network_get_dnssearch()
	  by passing "true" as optional last argument
	- internally cache fetched values to speed up subsequent accesses to the same
	  data, introduce network_flush_cache() to clear them
	- add some inline function documentation

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34722 3c298f89-4303-0410-b956-a3cf2f4a3e73