install: Install manpages into /usr/share/man

Currently we install manpages into /usr/man but they end up in
/usr/share/man anyway, because of the /usr/man -> /usr/share/man
symlink.

The problem is, that the bee index is wrong and, for example, `bee query`
doesn't identify the source of the files in /usr/share/man. According to
FHS [1], /usr/share/man is the correct destination.

Fix install script to use /usr/share/man instead of /usr/man.

[1] https://refspecs.linuxfoundation.org/FHS_3.0/fhs-3.0.html

install.sh

@@ -21,7 +21,7 @@ fi
 : ${usr_exec_prefix:=$usr_prefix}
 : ${usr_bindir:=$usr_exec_prefix/bin}
 : ${usr_sbindir:=$usr_exec_prefix/sbin}
-: ${usr_mandir:=$usr_exec_prefix/man}
+: ${usr_mandir:=$usr_exec_prefix/share/man}
 
 : ${usrlocal_exec_prefix:=$usrlocal_prefix}
 : ${usrlocal_bindir:=$usrlocal_exec_prefix/bin}