Add API and mainloop option for server hostname aliases

[michaelrsweet]

Currently PAPPL validates the Host header against the currently configured hostname, however many systems have multiple hostnames ("mycomputer", "mycomputer.local", "mycomputer.example.com", etc.) CUPS provides a ServerAlias directive to allow for configuration of multiple hostnames or "*" to allow any hostname.

Add system API:

extern void papplSystemAddHostAlias(pappl_system_t *system, const char *name) _PAPPL_PUBLIC;

Also add PAPPL mainloop option -o host-aliases=NAME[,...,NAME] to add aliases.