socket: support binding to interface *AND* IP

Introduce new notation for CURLOPT_INTERFACE / --interface:
ifhost!<interface>!<host>

Binding to an interface doesn't set the address, and an interface can
have multiple addresses.

When binding to an address (without interface), the kernel is free to
choose the route, and it can route through any device that can access
the target address, not necessarily the one with the chosen address.

Moreover, it is possible for different interfaces to have the same IP
address, on which case we need to provide a way to be more specific.

Factor out the parsing part of interface option, and add unit tests:
1663.

Closes #13719

.github/workflows/proselint.yml

@@ -51,7 +51,7 @@ jobs:
           JSON
 
       - name: check prose
-        run: a=`git ls-files '*.md' | grep -Ev '(docs/CHECKSRC.md|docs/DISTROS.md)'` && proselint $a README
+        run: git ls-files '*.md' | grep -Ev 'CHECKSRC.md|DISTROS.md|CURLOPT_INTERFACE.md' | xargs proselint README
 
       # This is for CHECKSRC and files with aggressive exclamation mark needs
       - name: create second proselint config
@@ -68,4 +68,4 @@ jobs:
           JSON
 
       - name: check special prose
-        run: a=docs/CHECKSRC.md && proselint $a
+        run: proselint docs/CHECKSRC.md docs/libcurl/opts/CURLOPT_INTERFACE.md