tests: cidr.sh: Fix for quirks in RHEL's ipcalc

tests: cidr.sh: Fix for quirks in RHEL's ipcalc

RHEL ships a special ipset[1] tool with different output in corner-cases
than the common one[2]:

* Reduced output with /32 netmasks:

| # ipcalc 255.255.255.254/32
| Address:	255.255.255.254
| Address space:	Reserved

To cover for this, make net_last_addr() fall back to the 'Address:'
line. Simply adding this keyword is fine as in normal output it appears
first and thus the other recognized keywords' values take precedence.

* No "Address:" line with all-zero addresses:

| # ipcalc 0.0.0.0/1
| Network:	0.0.0.0/1
| Netmask:	128.0.0.0 = 1
| Broadcast:	127.255.255.255
|
| Address space:	This host on this network
| HostMin:	0.0.0.1
| HostMax:	127.255.255.254
| Hosts/Net:	2147483646

Have net_first_addr() fall back to the 'HostMin:' line in this case.

[1] https://gitlab.com/ipcalc/ipcalc
[2] http://jodies.de/ipcalc

Fixes: e24e7656b3dd9 ("tests: cidr.sh: Add ipcalc fallback")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>