libkmod: modinfo: use own function for sig_key hex output

libkmod: modinfo: use own function for sig_key hex output

Refactor the code a bit to make it easier to extend for signature
output.

kmod_module_get_info() creats a hex string for the sig_key data
inplace. Separate it into own kmod_module_hex_to_string function
and handle the branch in the new kmod_module_info_append_hex,
keeping the same signature as the non-hex version.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>