Refactor protocol literals into constants

[JSYoo5B]

Protcol string for PGM, EPGM and NORM is still represented in literal strings.

While changing literals into constants, I also applied conditional compile for them.
Some code blocks didn't covered with conditional compile, and the others is using both PGM and NORM.
I tried to change those code blocks as simple as possible, but some code blocks seems messy.

I want someone review and comment especially for following code blocks. (I've marked WiP just for review)
Code block1
Code block2
Code block3

[JSYoo5B]

Added new commit to resolve previous ifdef tricks, rebased from master branch.

[bluca]

there's a formatting problem

[JSYoo5B]

Fixed formatting issue, ammended in last commit

[bluca]

still errors:

--- a/src/socket_base.cpp
+++ b/src/socket_base.cpp
@@ -1038,17 +1038,16 @@ int zmq::socket_base_t::connect_internal (const char *endpoint_uri_)
     //  PGM does not support subscription forwarding; ask for all data to be
     //  sent to this pipe. (same for NORM, currently?)
 #if defined ZMQ_HAVE_OPENPGM && defined ZMQ_HAVE_NORM
-    const bool subscribe_to_all = protocol == protocol_name::pgm
-                                  || protocol == protocol_name::epgm
-                                  || protocol == protocol_name::norm
-                                  || protocol == protocol_name::udp;
+    const bool subscribe_to_all =
+      protocol == protocol_name::pgm || protocol == protocol_name::epgm
+      || protocol == protocol_name::norm || protocol == protocol_name::udp;
 #elif defined ZMQ_HAVE_OPENPGM
     const bool subscribe_to_all = protocol == protocol_name::pgm
                                   || protocol == protocol_name::epgm
                                   || protocol == protocol_name::udp;
 #elif defined ZMQ_HAVE_NORM
-    const bool subscribe_to_all = protocol == protocol_name::norm
-                                  || protocol == protocol_name::udp;
+    const bool subscribe_to_all =
+      protocol == protocol_name::norm || protocol == protocol_name::udp;
 #else
     const bool subscribe_to_all = protocol == protocol_name::udp;
 #endif

[JSYoo5B]

I thought only tab->space problems. I just realized using clang-format.

clang-format patches applied.