Allow -onlynet=onion to be used

Just an alias for onlynet=tor, but matches the new name
of the proxy option -onion= better.
Add IsReachable(net) function

Allows other parts of the program to query for reachable
status of a network. Similar to IsLimited(net).
Add GetNetworkName function

Returns the network name for an Network enum.
build: Fix boost build on some platforms

When the libpath doesn't line up with the value from config.sub, we don't find
the correct path to boost's libs. This adds a hack to try another path before
giving up.

Should close #3219.
doc: Remove outdated information about boost versions

Bitcoin core should work with any remotely recent boost version
if a proper build environment is present. Remove a confusing comment
from the build documentation.
Add detailed network info to getnetworkinfo RPC

This commit adds per-network information to the
getnetworkinfo RPC call:

- Is the network limited?
- Is the network reachable
- Which proxy is used for this network, if any

Inspired by #2575.
Merge pull request #4605

aa82795 Add detailed network info to getnetworkinfo RPC (Wladimir J. van der Laan)
075cf49 Add GetNetworkName function (Wladimir J. van der Laan)
c91a947 Add IsReachable(net) function (Wladimir J. van der Laan)
60dc8e4 Allow -onlynet=onion to be used (Wladimir J. van der Laan)
qt: Use quint64 for formatServicesStr

`uint64_t` was causing a build error on some systems, as that type is
not known after including just the Qt headers.
Revert "build: add option for reducing exports"

Revert #4663 for now. It still breaks the pulltester.

This reverts commit 4975ae1722cd8af63eda2f02ef64a98091b6fb58.

Conflicts:
	configure.ac
Merge pull request #4704

54c7df8 build: Fix boost build on some platforms (Cory Fields)