util: add parseint32 function with strict error reporting

None of the current integer parsing functions in util
check whether the result is valid and fits in the range
of the type. This is required for less sloppy error reporting.
Use new function parseint32 in SplitHostPort

Use the new function parseint32 in SplitHostPort instead of calling
strtol directly.
net: Add CSubNet class for subnet matching
rpc: Use netmasks instead of wildcards for IP address matching

`-rpcallowip` currently has a wacky wildcard-based format. After this
commit it will accept the more standard format, for example:

- Ranges with netmask 127.0.0.0/255.255.255.0, ::/0
- Ranges with cidr 12.3.4.5/24, 12:34:56:78:9a:bc:de:00/112
- Loose IPs ::1, 127.0.0.1

Trying to use the old *?-based format will result in an error message at
launch.
Remove unused function WildcardMatch

No longer necessary after implementing netmask-based matching.
Also remove a longer-unused function `skipspaces`.
Add tests for BoostAsioToCNetAddr
Merge pull request #4102

21bf3d2 Add tests for BoostAsioToCNetAddr (Wladimir J. van der Laan)
fdbd707 Remove unused function WildcardMatch (Wladimir J. van der Laan)
ee21912 rpc: Use netmasks instead of wildcards for IP address matching (Wladimir J. van der Laan)
e16be73 net: Add CSubNet class for subnet matching (Wladimir J. van der Laan)
d864275 Use new function parseint32 in SplitHostPort (Wladimir J. van der Laan)
0d4ea1c util: add parseint32 function with strict error reporting (Wladimir J. van der Laan)