[fpc-pascal] Get user ID for name on Linux

[Marco van de Voort]

In our previous episode, Graeme Geldenhuys said:
> As Marco said, you can use the "users" unit. Here is an example of how I 
> use in in fpGUI, and it supports 32-bit and 64-bit systems.
> 
>  
> https://github.com/graemeg/fpGUI/blob/develop/src/corelib/x11/fpg_x11.pas#L4010-L4044
> 
> Note though that it only works for Linux. So other UNIX-type systems 
> like FreeBSD and Solaris are not supported.

FreeBSD is mostly supported,  though two units (shadow and crypth) and some calls
relating to shadow passwords in users are not.