ext library to share common functions

[susi]

Many of the external commands have identical or very similar functions, like for instance how LS , MV and SZ parse the given file names, or CP and MV should behave identically. There is probably also more code which could be shared.

Many commands could also share a common command line parser.

Proposal is to extract these identical or nearly identical functions and put them in a shared library ext.lib, built from ext.c