[gpm] Check return codes everywhere

[Markus Elfring]

Hello,

Some checks for return codes are missing.

Examples:
Would you like to add more error handling for return values from
"strdup" like in the function "parse_argv" and from "fprintf" in the
function "check_uniqueness"?
http://unix.schottelius.org/cgi-bin/gitweb.cgi?p=gpm;a=blob;f=src/mice.c;h=dacee3474f30919292a1cc22a1468e993bc61a38;hb=HEAD#l157
http://unix.schottelius.org/cgi-bin/gitweb.cgi?p=gpm;a=blob;f=src/daemon/check_uniqueness.c;h=4972413f6706bd491453ea862eae4c60af09ed32;hb=HEAD#l47

Regards,
Markus