astraceroute: Use size_t for struct members storing packet lengths

Packet lengths are strictly positive and the functions dealing with them
(sendto(), recvfrom() etc.) take size_t, thus use size_t.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
dev: Make device_mtu() return size_t

We don't return negative values and the MTU is supposed to be the
(maximum) length of a packet, thus make it of type size_t. Most of the
users in the code store it in size_t already anyway.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
astraceroute: Fix compiler warnings with "-W -Wall -Wextra"

Fix the following compiler warnings that occur when building with "-W
-Wall -Wextra":

astraceroute.c: In function ‘__assemble_data’:
astraceroute.c:255:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
astraceroute.c:262:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
astraceroute.c: In function ‘check_ipv4’:
astraceroute.c:425:56: warning: unused parameter ‘ttl’ [-Wunused-parameter]
astraceroute.c: In function ‘handle_ipv4’:
astraceroute.c:451:49: warning: unused parameter ‘len’ [-Wunused-parameter]
astraceroute.c: In function ‘check_ipv6’:
astraceroute.c:511:49: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
astraceroute.c:491:56: warning: unused parameter ‘ttl’ [-Wunused-parameter]
astraceroute.c: In function ‘handle_ipv6’:
astraceroute.c:517:49: warning: unused parameter ‘len’ [-Wunused-parameter]
astraceroute.c: In function ‘__process_node’:
astraceroute.c:690:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
astraceroute.c: In function ‘__process_time’:
astraceroute.c:738:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
astraceroute.c:755:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
astraceroute.c:765:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
astraceroute.c: In function ‘__process_ttl’:
astraceroute.c:846:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
astraceroute.c: In function ‘main’:
astraceroute.c:1060:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>