bash, bash_static: version bumped to 4.3 patchlevel 18.

For the bash (and bash_static) module from time to time there are official patches released. 
 I have created a way to simplify the update of the module(s) and have a minimum acceptable security check. I have added in the DETAILS file the RSAKEY of the dev who sign the patches (i presumed it's the same person for all of them - i have to keep an eye on this assumption), and the PATCHLVL value. 
 I have added a PRE_BUILD file in order to download the RSA Key needed to verify the patches, download the patches, verify them against RSA Key, apply the patches. I wanted to assure an acceptable level of security (which depend on 2 factors, 1. being connected to the Internet and 2. the patches and their signatures to be trusted on the source url). Maybe we need to mark the patchlevel value in the installed module name somehow. I have been inspired by the arch linux module(s).
 Please verify the proposal and feel free to improve it.