Install manpage in share/man/man1 instead of man/man1

According to FHS 2.3 the correct place to install the manpage for a
binary in /usr/local/bin/ would be /usr/local/man/.

However, on Debian/Ubuntu-based systems /usr/local/man/ is a link to
/usr/local/share/man/ and on other systems /usr/local/man/ might not
exist at all.

Even worse, if the client is installed in /usr/bin/ the manpage
should should be installed in /usr/share/man, but will be installed
in /usr/man/ which is completely wrong.

This patch fixes this and uses share/man/man1 as common prefix. Doing
this will install the manpage either in /usr/local/share/man/man1 or
/usr/share/man/man1.

Partial-Bug: 1269715
Change-Id: I590932f00476eacd434cdae012fd62010845581d

setup.cfg

@@ -27,7 +27,7 @@ packages =
 scripts =
     bin/swift
 data_files =
-    man/man1 = doc/manpages/swift.1
+    share/man/man1 = doc/manpages/swift.1
 
 [build_sphinx]
 source-dir = doc/source